You can cancel anytime.
There's no etf's anymore. If you finance a phone you would need to pay it off though.
To get a "normal" (postpaid) account, you'll need a US credit score. So, a prepaid one is probably a better choice.
T-Mobile does not prorate partial months, so when you cancel your account, do it just before the end of your service month.
There is also the no credit check option still I believe.