Use only those with 'Taxi Meter' sign on roof. Check that meter is started only after you enter. Flagfall is 35 Baht, for first two kilometers. Pay extra for expressway tolls, at cost. Take with you a card from the hotel with its name written in Thai to make the return trip easier.
Public buses are cheap and follow set routes.
A map is available from book stores showing routes. Drivers do not usually speak English.