All journeys are metered, and you should expect to pay roughly 120 ZAR for a trip into the centre. Ordinary taxis are metered and you can hail them from the street. You will do better if you go to a taxi rank. Taxi ranks are situated in Greenmarket Square and the Tourist Rendezvous Travel Centre in Adderley Street. Black taxis, which are usually not black and are usually minibuses, in general follow major bus routes and pick up and set down at bus stops.
Buses are somewhat cheaper but also somewhat slower.
Around Town on Foot Central Cape Town is quite small and you can get around perfectly well on foot. It is hard to get lost since you should always be able to see Table Mountain from which to take your bearings.