Yellow shared taxis are the standard way of getting around — flag one down, state your destination, and the driver will take you with whoever else is going the same way. Fares are by zone and small; a private hire ("dépôt") for the whole car costs several times more.
The Trans'Urb city buses are cheap but slow and unreliable. Most foreigners simply use taxis. There are no metered cabs; agree the fare before getting in if you are taking the taxi alone.
Walking is fine in the city centre by day. Traffic gets heavy in the morning and evening rush.