Most of Ngaoundéré's centre is walkable — the palace, mosque, market and main hotels are within a kilometre of each other. For longer trips, yellow shared taxis run fixed routes for a small fixed fare; flag one down and shout your destination. Motorcycle taxis (clandos) are faster but riskier; agree the fare before climbing on.
For the outlying lakes and the airport, you will need to hire a taxi for the trip or take a shared minibus from the central park.