Can I use unlocked Iphone 4 with AT&T Prepaid?

Ok, so I just ordered an unlocked Iphone 4 online. I thought I could simply put an AT&T sim card in it and buy the $50 prepaid card and have unlimited talk, text and data. Now, I'm hearing that you can't use data on AT&T's prepaid service. But, I also read you could change the APN settings via safari and data will work. Can someone explain to me how this works and how I can get unlimited talk, text and data on a prepaid plan through AT&T or another service. I heard about straight talk but I don't think that will be an option for me because of where I live. Thanks in advance.

Added (1). One more question. How can I get the sim card? Do I need to buy it or is it free?

You can use it on AT&T GoPhone prepaid, but not the $50 plan. You'll need to get the $65 a month unlimited talk, text plan with 1 gb of data.

Answer to your edit… They gave me my sim card, but I do think they can charge. Go to the website and see how much they charge for a sim card. I took mine to the store and was talking to them and they were real nice. I didn't have to spend a dime except for when I bought my airtime.