Switching from an IPhone 4S to 5?

So I'm buying an iPhone 5 off of my friend and I have AT&T. It's an AT&T phone. How do I switch the SIM cards? Or do I not have to? Because I'm aware that they have different cards & if I do should I buy it at the AT&T store? How much will it cost?

Go the AT&T store they will give you a new sim card free of charge and they will swap it in.

You would have to either cut your microSIM to nanoSIM size or have AT&T issue you a nanoSIM with your number on it. Yes if at all possible switch the SIM of course since that is the one with your number on it.