How to use and Iphone 4 with t-mobile?

[holllllllerwatsuuuuuupt:]

i have a t-mobile phone and im on the unlimited data plan, so it has internet and all of that on it. my friend offered to sell me her iphone 4 for fifty bucks, its in great condition just the back has a crack on it ( I go to school with a bunch of paris hiltons ) hahaha. so she doesnt use it anymore and got a new one. anyways, i have the money but if i buy it then what do i do? like as far as sim cards and everything goes? im not sure what to do at all after i get it so can anyone help?