What other carriers can I use with a T-Mobile SIMs?

I know that I can use AT&T phones if it's unlocked.. but I dunno how to unlock it.. Or, rather my friend unlock it. I need a new phone because mine is very very crappy - Gravity 3, 2 years old, and freezes daily. So I'm buying a LG neon off of her from AT&T but she doesn't know how to unlock it either.

-How do I unlock the neon?
-What other carriers can I use if the neon ultimately fails?

Thanks<3