i have a cell phone already from tmobile. if i bought a nother tmobile cell on Ebay or somewhere would i be able to have like my same number and all that crap on it. and how much would it cost?

All you have to do is take your SIM card out of your current phone and put it in the other phone you bought from wherever. It'll work just fine, no problems. You won't need to change your number and the only cost you'll be paying is that for the new phone you're buying.