Pragues's Ruzyne airport (code PRG) is located about 10 kilometers or so from the city. It is a relatively small airport, and it's very easy to naviagate within the buildings, consisting of one arrivals and one departures terminal that are connected by a food court.
There are a few options for getting from the airport to the city: there are minibuses that go to hotels and can be a good deal for several people sharing, there are taxis, and there is public transport. Taxis have been known to rip people off. In fact, they are famous for it. It is recommended that you avoid this option. The minibuses are worth looking into.
However, for travellers on a budget, Prague's nicely integrated transport system is a very good alternative. All you need to do is head outside from the Arrivals area, and go to the bus stop across the street for Bus 119, which goes to Dejvicka (pronounced Dey-vitz-ka). Take the bus to Dejvicka (about a 20 minute ride) and get off. There, you'll find an entrance to the underground trains, about thirty feet ahead of you. Dejvicka is a little bit separated from the city center, so most people will need to connect to the underground train. This allows for a quick ride directly into the city center, with connections to just about wherever you'd want to go.
A ticket for the bus can be purchased at the desk located on the other side of the Arrivals hall, slightly to your right when you come through past immigration. A 12 Crown ticket (50 cents US) will give you an hour from the time of stamping the ticket, which should be plenty of time. On the bus, you'll need to stamp the ticket. The ticket will be good for 60 minutes (90 minutes on weekends) from that point on all buses, trams, and underground trains in the city.