my AT&T account

I bought the phone from somebody I knew they bought it brand new but can't remember the password or email address that they use to set up the my AT&T account how can I set up my the account for me and not them how do I remove all their stuff and replace it with mine as far as the account goes I don't care about the number being changed or anything like that I just want to be able to see my balance and so on so forth and change payment options