IPhone 4 Straight Talk Sim Card

I want to order a sim card for my iPhone 4, but I don't know which one to choose or if it even matters.
So when I go to buy the sim it asks if it's a CDMA, unlocked GSM, AT&T compatible or T-Mobile compatible.
My iPhone isn't unlocked because I heard you don't need to, and it was with AT&T beforehand, so which do I choose?

You want the AT&T. Then, select Micro Sim. Then, choose the plan you want.