Prague has amazing public transport system, it is clean, convenient, and cheap way to get from point A to B, including from airport to city center. Prague public transport includes buses, trams, subway (metro), and some trains, boats, and also a funicular.
Tickets for Prague public transport can be bought in yellow machines, directly in trams and some busses, and more conveniently via PID Litacka app available for both iPhone and Android: https://app.pidlitacka.cz/
In the application you want to select Tickets > Buy a ticket > Prague
Tickets start at 30 CZK per single 30 minute ride for adult, but we recommend 330 CZK (14 EUR) 72 hour ticket, or 120 CZK (5 EUR) 24 hour ticket, if you plan to explore Prague a little bit.