You can take a taxi from outside Prague airport directly to the city centre. It takes around 20-25 minutes and should cost around 20-25 Euros.
Alternatively you can take bus 119 to Nadrazi Veleslavin and then take the metro line A to the city centre (direction Skalka/Nadrazi Hostivar). The ticket for adult is for 32 CZK (1.2 EUR) and is valid 90 minutes.