How to switch AT&T iPhone 5 to Straight Talk, Keep old Straight Talk number?

I bought a used AT&T iPhone 5 off eBay today for my girlfriend. I also bought a NANO Sim for Straight Talk. She wants to keep her old straight talk number. How do I switch everything over so data, mms, etc works and she keeps her number? There's lots of different info out there right now, I just want some definitive instructions on how to make the switch properly.

Unless the iPhone is unlocked, you can't use that sim card in it.