Praise the sun. Our Dark Souls Walkthrough provides tips for surviving the adventure, a guide to the entire world, and hints for slaying bosses.
Shopping and Forging
Souls are not only your means of improving stats, but also the currency in Dark Souls. Merchants and smiths will offer their services for your hard earned Souls, but most aren't interested in buying old equipment. Keep this in mind, as spending a few thousand souls on a neat piece of gear might be a waste if you can't resell it.
You'll eventually be able to sell items for Souls to one important NPC, though they only appear in Firelink Shrine after completing the game's first main objective: ringing the two bells and opening Sen's Fortress.
Some NPCs (typically ones that have already set up shop) will remain stationary, requiring you to travel in order to purchase their goods. Vendors that start captured and are rescued on your adventure will often journey back to the safety of Firelink Shrine.
The different retailers carry various amounts of items, including consumable items, gear, keys, and spells. Some of them are distrustful of Hollows, and may require you to be in Human form to sell items. Others are wary of outsiders and will only deal with members of their Covenant. Listen to the dialogue of an NPC to get an understanding to why they are avoiding business.
Other NPCs might be outright hostile to Hollows and rival Covenants. By changing into a Human form or joining another organization might turn an old foe friendly. Likewise, avoid outright attacking NPCs, as many will fight back, rending their services unavailable.
Blacksmiths around the world can help repair, reinforce, and modify equipment. Each piece of your equipment has durability, and as they are used or hit, will require Souls to repair. With some materials of your own and a grinding stone, you can also do some repairs at a Bonfire.
Most basic pieces of weapons and armor use Titanite to increase damage and defenses. More complex items might require other stones, or even exotic Embers to customize your weapon. A good smith will also trade you such materials for Souls. Of course a blacksmith's services are not free and may demand several hundred Souls for the labor. Still, it is generally better to upgrade old gear instead of buying basic wares from other merchants.
Each of the smiths specializes in a different ascension and only accepts embers they are comfortable with. If the blacksmith down in the Undead Parish doesn't want to work on Crystal or Flame weapons, consider donating the embers to the smith within Anor Londo.