Unlocked Cell Phone for Tmobile?

[Stacey W]

I am not on contract will tmobile I pay retail for my phones which is expensive.I am interested in purchasing a phone from Ebay and some are unlocked phones that are compatible with tmobile. How does that work? do I simply put my sim card in ? I plan on getting a blackberry or some sort of android or smart phone will I be able to still purchase a data plan with t-mobile even though its not their phone