Is there a down payment for T-Mobile prepaid service?

So I have an AT&T iPhone 4 thats unlocked and I plan to use it on T-Mobile's prepaid service thats $50 a month. I'm wondering when I go to activate the phone and service how much would I have to pay upfront? I would need a sim card. Does the sim card come free? Would i pay the bill upfront? Or would I just pay the fee for the sim card and at the end of the month pay the bill?

All these question could be answered by a quick call to t-mobile customer service., don't be afraid t call a customer service number dude. Be brave, all t-mobile with your ATT phone.

I think you will only get 2G (Edge) if you use an ATT iPhone 4 on t-mobile, but do ask them about that.