Simple, you go into your user CP at the top left, and find the edit signature option along the left side.
As for rep, you get it from other members. How much you get is their rep power. Mine is 9. You get 1 rep power for every year as a member, 1 for every 1000 posts, and one for every 500 rep points you got from other members.
Admins dish out 50 reps each. So make sure to suck up and make sure they see your mod threads! lol