Do I need to take Iphone to AT&T?

I just got it off Ebay, but my old phone just has a regular sim so will I have to take it to the store? I looked into buying one online but I already have a data plan or whatever so I don't know how that would work. I can't really seem to find a clear answer.

You don't have to take the phone. Just tell your provider you want a different size SIM. Att usually gives them for free. I buy all my phones online. I just ask for a different size SIM.

If your current SIM card won't fit in you have to take it to AT&T and get a replacement.

Your regular SIM needs to be trimmed to microSIM (iPhone 4, 4S) or nanoSIM (iPhone 5 and above). Take your SIM to your carrier and they'll trim it.