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 (last stop) and then take the metro line A to the city centre (direction Skalka/Nadrazi Hostivar). The nearest stop is Mustek. From here you walk along the street Na Prikope and Revolucni (still straight). In Revolucni street you take the first turn left to Dlouha Street. Then you walk 3 blocks straight and on the left side you will see number 24 with open yard and bikes on the street. The metro ticket is for 32 CZK (1.2 EUR) and is valid for 90 minutes.