The forest covenant is located inside the Darkroot Garden. There are two ways to reach the covenant. There is a back entrance, that you can find by following the shore harboring the Hydra (Near the giants, and the Havel tower). Alternatively, you can buy a seal from the blacksmith in the Undead Parish area to unlock the large sealed door, which blocks off the covenant area; the seal costs 20,000 souls.
You should know you are close to the covenant, when you enter an area with human enemies. After they are dispatched (or avoided), you need to enter the crumbling stone building that they gather around. It should be easy to spot with a little exploration around the area. Inside the building, you should see a strange rat/cat monster sitting in the windowsill. Talk to it, and respond with yes. You should then be asked if you want to join the covenant. Achievement unlocked!
You can enter the Darkroot Garden area, through the large doorway adjacent to the Undead Parish blacksmith. The area is guarded by a Titanium Demon.