Q: Do I have to go to my carrier if i wanna switch my iphone?

You need to contact the carrier in this case AT&T to setup your own account and since you are purchasing the iPhone from a friend, you will have to get permission to change the Apple ID.

al__ wrote:

 

My friend is selling his iphone 6s  so I was wondering if I could just put my sim card in it, ya the phone has the same carrier as mine. Can i just switch the sim cards or do I have to take it to at&t?

Yes, you should just be able to put your current active SIM in a the new phone. No need to talk to AT&T.

As Carolyn notes, make sure your friend has signed out of their Apple ID. Show them this article:

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support