Alice11 has just about told you all you need to know.
UA arrives @ the satellite terminal. From there you catch a tram to Immigration .
Get in the non EU citizen line ( I assume you are a US Citizen ) then you'll have a long hike past baggage claim where you will exit Customs. If you don't have anything to declare , just walk right out the door.
You'll exit pretty close to Terminal B.

You can get a map of FCO. go to gofox.com click on travel tools then Airport maps . Gives you a pretty good idea of the terminal layout.

BW