For this cheat go to Megaton "the first city when you exit the vault".

Go to the atomic bomb in the middle of Megaton. Now talk to "Confessor Cromwell", there should be a diolouge "Do you accept donations?".

If there isnt then choose dialogue "Tell about the chruch of atom" and ask "what do you believe in?", then go back and then do "do you accept donations?".

You may then choose "10 caps, 50 caps,or 100 caps" - each time you donate you will recieve good karma.