*UPDATED TO REFLECT CHANGES FROM THE NEW PATCH!
Achievement description- Covenant: Warrior of Sunlight
To get this achievement, you need to join the covenant of sunlight. To do this, you have to have at least 25 points in the faith stat.
Or there is also a catch. If you are lucky enough to have played in multiplayer enough times, you can also reach above 25 quite easily. Every time you are the client in multiplayer, you will gain 5 points toward the 25 required to join the covenant. For example, if your faith stat is only a 10 but you have been the client in multiplayer 8 times, you can still join the covenant. (10+ (5x3) = 25)
In order to actually find this covenant, there are a few ways. One way is to contact the Warrior of Sunlight Solaire (the guy on the bridge you met in Undead Burg who lets you summon him) with a stat 25 or greater, and he will ask you to join his covenant. You can also encounter him later in the game in Anor Londo.
If for some reason you can't find Volaire, you can go to the Sunlight Altar which is conveniently across the bridge that the Red Dragon is located on. If your HP is high enough, you can make a run for it and pray you don't die getting there. :) Make sure you equip a fire resistant armor too! Once you reach the bonfire inside that castle across the bridge, turn right to go slightly outside and you'll see the altar. If your faith stat is 50 or above, the altar will also allow you to join the covenant. Good luck!