How to use a t mobile sim card on iphone 5?

I know its called jailbreaking right? I'm gonna get an iphone this weekend and i just want to know what are the risks if i don't do it right. How do i jailbreak it? Better yet, what are the steps to making my t mobile sim card work with it? Do apple employees let you buy the phone itself without contract to at&t? If i told them i'm going to use it but not with at&t, would they allow me to buy it?

You need to buy it unlocked or buy it without a contract, two different prices.

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