How do I terminate my T-mobile contract?

It depends on which contract you're asking about. All T-Mobile service contracts are month-to-month, so you just let them know you don't want to continue. T-Mobile does not prorate partial months on cancellations, so make the effective date at the end of your billing month.

If you have an EIP contract, that will need to be paid off. You should also request any T-Mobile devices be unlocked, so you can use them with a local SIM in your new country.