I bought a used T-Mobile iPhone 6

I bought a used T-Mobile iPhone 6, help?!

Added (1). Everything is fine with it, i'm just worried as to changing my sim card. Can I take my sim out of my current iphone and put it in the new one on my own without involving t-mobile or do I need to call them or something? I'd like to just take care of it on my own.

MAH WORLD IS OVA I don't know WHAT TO DO I don't know WHAT I'm DOING
please provide info.

What do you need help with?