Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
New Londo Ruins
You can access this area from the beginning of the game, but it appears that you're not really expected to fully explore it and clear it until after you have cleared Anor Londo (accessible after you beat the Iron Golem in Sen's Fortress). You may want to venture into the area anyway just because there are some great items that make the Curse status effect less of an issue elsewhere in the game.
In Firelink Shrine, there's a man who sits on the stone wall near the bonfire. Just past him, a trail descends to a graveyard area. You can find where there's a cave with bars across it (the place where you can upgrade your estus flask once you have the proper item) and then you can head down some stairs from there to reach a room with a lift. There's a lever if you just see a gaping hole. Otherwise, step on the center of the lift to ride it down to a lower level.
Once you arrive at that lower level, head out and to the left, then descend along some stairs. There are weak enemies here in the graveyard that you can attack as you search for soul items. Keep descending and you'll soon see notification flash on-screen to let you know that you have entered the ruins.
From the entrance to the ruins, note that you can head to the right and then down some steps that descend along the side of the cliff to find a blacksmith in a cave. He'll help you through the bars.
Otherwise, head toward the left from the area entrance and look for three skeletons supplicating themselves against a large stone wall. There's another single skeleton to their left, and to the left of that an opening in the stone wall. You can pass through that opening and look sharply right to find the Estoc weapon resting on a corpse. From there, you can descend toward the left to reach the shore of a large body of water. You can head across the narrow wooden platform and along stone islands to reach an area that is haunted by ghosts.
Note that ghosts are not weak to your standard physical weapons. You can only harm them if you are cursed (the Transient Curse item is useful in this regard, since it temporarily curses your character without the need to find an expensive weapon and remove the curse) or if you have a special weapon that only the ghosts drop. It's not a bad idea to bring a bunch of Transient Curse items, then, and farm ghosts near the ruin entrance until you obtain that weapon, the powerful Jagged Ghost Blade. Then you'll fare better against any future ghosts, even if you aren't under the effects of a curse (though you should upgrade it with a blacksmith before you head deeper into the ruins).
When you're ready to head into the ruins properly, start forward across the shallow pool of water that you gain access to after wandering along the docks. Some ghosts will hunt you down here, but you should be able to take care of them easily since they're out in the open. Then you should look around at the stone walls, near the edge where the water grows deeper. You will find a corpse in a barrel behind a low stone wall. Roll through the barrel to break it open and knock down the corpse. You will receive two Transient Curse items as a reward.
If you look around in that general area, you should see a distant glowing point that represents an item on a corpse. It appears to be floating on the deep water, but you can still reach it. There's a barely-submerged stone ledge that leads out there. As you near the item, two ghosts will rise from the water. Don't let them knock you from the ledge as you hack at them and kill them. You might just want to roll through them so you can fight them on more solid ground. Be careful about fighting in the doorway, too, as the camera likes to shift so that you press one direction and your character walks another (usually to his or her death). When you defeat the ghosts (or before), check the nearby corpse to receive the Fire Keeper Soul that you can trade in for another estus flask upgrade.
Now head back to the shallow pool of water and start uphill. In this area, almost any corpse you check is going to obtain more Transient Curse items (a good thing, since you need it to be able to be offensive here). You should be able to stockpile enough to survive for awhile, since the effects of the item last a fair while. Keep an eye on yourself, though; you don't want to have the effects wear off just before you need to kill some ghosts.
As you head up the hill from the shallow, low-lying water areas, you'll come to a series of stone rooms. Here, the ghosts will come at you a great deal more thickly. Head up the steps and across the bridge ledge, but be careful. Ahead of you, as you start forward because a ghost is waiting in plain sight, another ghost will try to ambush you from a passage to the left. Wait for that attack and block it, then kill the ghost before you head forward to the next one.
In this area, you'll again be ambushed, this time by a bunch of ghosts that come at you from all sides. Be ready to roll around to avoid them and try not to let yourself get mobbed or struck from behind. Continue forward from there and follow the passage left to the top of a spiral staircase. Descend it.
From the base of that staircase, start forward and you'll see a corpse lying on the edge of a ledge. Approach carefully. There's a ghost hiding in the wall to its left. Get near and the ghost will sweep out its scythes. Attack from the side and kill the ghost, then check the corpse to obtain the Parrying Dagger.
Head back the way you came now, but take a left along a passage that you come to ahead of the base of the spiral staircase. Along that route, you'll find a mist-shrouded door. Before you enter it, you can head past it to find a ghost on a ledge near a corpse. It will scream as you approach. Kill it and then check the corpse for two more Transient Curse items. Then return to the mist-shrouded door and pass through it.
At the top of the stairs look forward and you'll see the top of a ladder. Make sure that you kick it, which creates a shortcut. Now if you have to come back here, you can leap over a gap from the shallow pool of water near the start of the area, then run along the ledge to reach the ladder.
From the top of that shortcut ladder, you can also head forward to fight a ghost and check a corpse (you'll receive the Large Soul of a Nameless Soldier), or you can head left. As you proceed left, and continue left at the 'T' in the path (right is just an obvious dead end overlooking the water), you'll come to a point where you're walking along the side of a building. Ahead, you'll see an opening that leads to the right. When you near that opening, ghosts will attack. Unless you're using magic, they can be difficult to fight without falling in the water unless you lure them into the building and dispatch of them there.
Once the ghosts are dead, you can head inside the house building and battle more of them. This isn't strictly necessary (you can run and jump along the outside portion of the building if you prefer), but it's a worthwhile detour because after you defeat the ghosts, you can climb a ladder hidden in a chimney. Make sure that no straggler ghost catches you by surprise (one will come out of the wall to attack as you start up the chimney).
If you make it to the top of the chimney, you'll meet a character named Ingward on the edge of the building's fenced roof. You can buy the Resist Curse spell from him for 5000 souls, if desired. That could come in handy elsewhere in the game. Depending on what else you have accomplished in the game (you need to have cleared Anor Londo and obtain the Lordvessel), he may also give you the Key to the Seal you need in order to proceed through the area. You can also kill him and loot the item from his corpse, if you prefer.
After meeting with Ingward, you can return to the lower path that led along the side of the building. Near the base of those stairs, make sure that you venture inside the building and head along the corridor to find the Cursebite Ring (which adds a welcome boost to your resistance to the Curse status effect) resting on another outside ledge along the building's side (there's a ghost guarding the corpse where you'll find it). In another hallway, you'll find the Soul of a Proud Soldier. Head back through the building and exit along the main ledge. Follow the ledge to find a low-lying area that looks like a stone gazebo along the left branch of a 'T' in the path. You can find a Green Titanite Shard on a corpse that you find there. Then head back to the 'T' and this time follow the steps that led upward.
At the top of the stairs, you'll find the Large Soul of a Nameless Soldier resting on a corpse. Follow the ledge left from there and you'll find some stairs ascending along a high stone wall to your side, as well as the entrance to another building ahead of you. Ignore the building for a moment and climb the stairs.
The stairs will lead you to Humanity on a corpse resting along the far end of the platform. You can also drop down from that ledge to land on the roof of the building where you met with Ingward. If you head along the eave, you'll find another corpse that you can check to obtain the Rare Ring of Sacrifice. Then you can drop into the building through one of the windows.
Now head back around the house and enter the final building that you previously ignored. You may not be able to do much just yet, depending on whether or not you have the appropriate key, but you can climb some steps to find a ledge overlooking a moss-covered walkway where you can pillage the Composite Bow and 16 of the Large Arrow. To do anything more in the area, you'll need to head downstairs and you'll need to have Ingward's key (the Key to the Seal) in your possession.
When you have the key, you can open the iron gate near the bottom of the area and then you can head along the passage and through another opening to find a gear like the ones you would have encountered in Anor Londo. You can grab onto the bar that extends from one side and give it a push. As the gear spins around, the nearby gates will open and high water will flood out of the area, draining it so that you can explore.
Head back the way you came and when you find the lever that you may have noticed before, give it a tug. You'll cause an elevator to rise up to your level, creating a handy shortcut to the Valley of Drakes. Now you can enter that region from the entrance to the New Lando Ruins and take an easy shortcut through that domain to reach the New Lando Ruins in the area near the lever that you pushed to drain the water. It can save a lot of inconvenient backtracking, as long as you're good at battling dragons.
Start through the area ahead of the elevator that you just descended and you'll encounter a new type of enemy. This fellow is ghostly but wears a suit of armor and wields powerful magic. You should attack him from a distance as possible using fire attacks. Try not to get too near to him, or to attract another of the enemies (there's another nearby). Defeat any of the nearby fellows, then head up some steps and to the right to find a small building. Inside, on a corpse at the end of a corridor, you'll find the Large Soul of a Proud Knight.
If you continue around this same general area, you'll find something a lot more troubling than just the one or two ghostly knights. As you head to the right, you'll find a handful of them. Try to lure them toward you one at a time and take care of them with your preferred method, all without luring more to the fight. You'll find two more Large Soul of a Proud Knight items as you work through the area. Eventually, you'll also reach a huge, black blob.
The blob is capable of absorbing a great deal of damage, and what's worse is that you'll likely be battling ghosts besides. Since the ghosts can be dispatched almost immediately, you should do so and then focus your attacks on the blob. It doesn't actually move fast at all, so you should be able to get in close and assault its flanks quite easily until it finally gives way. Then you can check the chest that it was guarding to find a Titanite Chunk.
In the same general vicinity, you'll also find six of the Cracked Red Eye Orb. Most important, though, are the items that you'll find along a high ledge above the area. To reach the ledge, climb some spiral stairs in the area that was most heavily guarded by the ghostly knights and the blob. Then cross along the chamber by following the narrow ledges that lead along the upper level (grab the Soul of a Brave Warrior along the way) and descend to find the Very Large Ember on a high ledge. Drop down and then head around again to reach the area near where you fought the huge blob. This time cross over a narrow ledge nearby to find a mist-shrouded doorway, which you should now enter.
The mist-shrouded doorway leads to the top of a very high staircase that descends in a spiral fashion through the cylindrical tower. Equip the Covenant of Artorias (a ring) and then head down to the base of the stairs. After preparing yourself, drop down into the blackness and you'll fall a long, long way into a new area known as the Abyss (which you can't leave until you either die or defeat the waiting boss).