From Václav Havel Airport, take the bus Nr. 119 to its terminal station Nádraží Veleslavín [naadrage: veleslave:n]. There you go down the stairs to Metro A (green line) and take it to Staroměstská station. Then it is just about 200 m walk.
The price should not be higher than 500 CZK. We recommend you to ask for the price before getting into the car and always ask for a receipt.