Do I just go to a tmobile store and get a SIM card for an AT&T iPhone 5s?

Do I need to do anything beforehand or should I just go buy a SIM card for the phone? I want it to be tmobile not AT&T. Nothing has been done with the phone with the exception of it being jail broken. If something does need to be done please tell and provide support

You just go buy a t-mobile sim (like $10) from the store, yes, Your AT&T iPhone must be unlocked to activate on t-mobile.

Not possible

Jailbreaking does not unlock a phone which is what needs to be done here. Chronic Unlocks should be able to help