Documents for foreigner?, Plan runtime?

Hello everybody!

I am a foreigner owning a SSN who will be working in the US for half a year. I would like to get the $50 individual plan for this period of time. I will bring my cellphone along to the US. Here are my questions:

1. Which documents do I need to supply except my passport and SSN to sign up for a plan?
2. Can cancel the plan on a monthly basis or do I need to stick with T-Mobile for a fixed time period, e.g. 24 months?

I am looking forward to your short answers! Thank you very much!