You want the TIM Welcome package. It costs €30 for the SIM, 2 GB of data (4G speed where available), 200 minutes of outgoing local and international calls, and €5 of credit for texts or more calls. The data cannot be renewed on this plan (if you run out, you'll have to get another plan), but you can top up the credit if needed for more calls or texts.
Not all TIM stores stock nano-SIM's, so be sure to tell them you need one at the outset; if that store doesn't have them, they'll direct you to one that does. TIM stores are all over, including in Fiumicino Terminal 1 (but that's the one that didn't have nano-SIM's, at least on April 28, 2014).
Receiving calls on this plan is free, regardless of whether the caller is in Italy, the US, or anywhere else. But calling from the US to your Italian phone will potentially cost more than calling an Italian land line, and your callers will have to know how to dial international numbers. One work around is text; have your callers text you instead (international texts are much cheaper than international calls), then call them back. Or if they're calling you a lot, your callers can look into an international package from their carrier, or a service like OneSuite: https://www.onesuite.com/long-distance/check-rates
