You will need to switch out your SIM for one that works in the US. Or it might be cheaper to get a lower quality phone to use in the US, and use you good phone on your German plan to call home, as well as using it with Wifi, wtc. Now - how are you just planning to stay for a year.? No permit to work, no green card, and you will overstay your tourist via. Unless you are already accepted into a university here, and have supporting paperwork, you will not be staying a year. More info please do we can help you.