Iphone 6 t-mobile free-contract?

Hey guys I'm from Kuwait and I'm traveling to colorado, usa to study there. I'm buying the iphone 6 in 28-29 sept. And I want to ask some questions about this t-mobile free-contract. So lets say if I buy it and I got it. What will come with the device? A t-mobile sim-card? And if yes. Can I change the sim card and use at&t sim card? Will it works? Or it must work in t-mobile sim card only? And if I traveled back to my country can I use sim card from Kuwait and will work?

and I what about the company it self? Is it good? My brother told me that it has bad service? Really?

Just ask them to unlock the phone if you want to hange the network.