how do i use a t-mobile phone with at&t?

[xxsuicidalcupcakesxx]

so a friend of mine has this phone that i really want she gave it to me but its tmobile
and i have at&t
so when i put in my sim card it asks me for a network key
i dont know what that is
so can anyone give me a hand here?
is there only one or many network keys or does it depend on the phone?
and if it doesnt what is the network key or how do i find it?
please help and thank you