Bicycle Tutor

My rear brake isn't functioning right. There is proper tension in the brake line and I replaced the pads, so I think that there is grease/oil on the metal where the pad hits. What is the best way to clean it? Or have I misdiagnosed?

The primary question is whether the adjustment was done right. If that's the case, did you "rough up" the pad surface with sandpaper? It would be good to double-check your work.
If you did get grease or chain oil on the rim surface, a simple degreaser works well.

I like to clean the braking surface of my rims with rubbing alcohol and superfine (0000) steel wool. Dampen a small pad of superfine steel wool with alcohol and clean any brake pad or other residue off the rim. Wipe the rim dry with a cloth. Make sure to use superfine steel wool -- it won't scratch or score the rim but more coarse steel wool may.