The easiest method for buying tickets at Metro stations in Rome is to use the automated Kiosks. They have an English language option, and are very easy to use. Especially at Roma Termini, buying tickets from an agent is not usually the best idea as there's usually a LONG queue. As Laurel mentioned, the Metro in Rome is very easy to use, especially as there are only two lines. One important point to mention - if a "helpful individual" tries to assist you in buying tickets at a Kiosk, I'd suggest telling him to "take a hike".
I normally buy a B.I.G. ticket (Biglietto giornaliero. €4) first thing in the morning and then use it all day. It's good for Metro, Buses, etc. within the city centre, and only has to be validated prior to the first use. It expires at midnight on the day of validation. There's also a three-day version called a B.T.I. (€11).
To figure out where you're going, there are charts on the walls of the Metro stations, or you can research your stops online. You may find it helpful to pack along a copy of one of the Guidebooks (perhaps the Pocket Rome) as that will be a good reference to use during your trip).