It's not a great idea to buy a Tim simcard in the US. I'm sure that you'll pay more than in Italy!
Also when you'll have a Tim simcard you can activate any kind of option via 4916 (IVR)... it's important only that you have enoguh credit in your balance. It's easy
Pay attention that you'll have "unlimited" navigation ony between 5 p.m. and 9 a.m. with that internet offer of Tim!