Can I use a used iPhone as a prepaid phone?

I have a used AT&T iPhone and I want to use it as a prepaid phone. It's my boyfriend's old phone and I'm going to use it because the phone I have now is old and slow. Can I activate this as a prepaid by myself or do I have to go to AT&T for that? He has a new phone, and he kept his same number, so would I have to activate the SIM card or buy a new one?

I know you guys aren't AT&T techs, but any information you could give me would help. If I have to go to AT&T, then I have to go and it's whatever, but I was going to try to activate it as a prepaid by myself through the website, if that's even possible.

Sure