How to change contract iphone to prepaid?

After talking with quite a few people I realized that I'm paying way too much money for my phone bill and I want to switch my iphone to prepaid. I currently have AT&T with an iphone 4s. So once I cancel my contract do I have to buy a new SIM card? I don't know if I'm going to go with AT&T prepaid… I might do Verizon or someone else

Added (1). Ah okay, I was leaning more towards straight talk so that's good. I'll just go to the AT&T store and tell them what I want to do.

You can not use a AT&T phone with Verizon.

You will need to use your AT&T Iphone with Straight Talk or Net10. You will have to get your Iphone unlocked through AT&T first.

AT&T will not allow an Iphone with AT&T prepaid.

Att does allow iphones on prepaid but its different… Att prepaid its 50 a month with no internet… And 65 a month for 1 gb of internet…

but if you unlock the iphone you can use it with tmobile, strait talk, net 10, h20, etc

I have a srpint contract phone and i want to change to iphone 4 prepaid for att i have dont have a sim card spot what do i do