Switching phones question

I currently have a Pantech Duo.
I like the phone, but overall it runs pretty slow and I seem to not get signal anywhere. I also want an iPhone.

Lately, I've been considering getting a white 16 gb iPhone. I just don't know what the process would be.

Could I go into Best Buy, buy the iphone, and just put the SIM card in?
Or would I have to go to AT&T and pay a deactivation fee and all that?

I really have no idea how to switch phones and what the cost would be, and whether it would be practical or not.

Please help me.

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Its 100% free all you have to do is switch the sim card