How do you get good karma very fast on fallout 3 on the xbox 360 version

Defuse the bomb in Megaton for one. Big karma gain and you don't have to do basically anything, and kill Burke while you're at it, more good karma there, if you lose karma by blowing up megaton, you can kill Tennpenny and Burke.
Or get the lawbrinbger perk. You get to work for the regulators and every bad person you kill you recover a finger and get paid 100 caps PER finger.