You can book a safari at most of the hotel receptions or local travel agencies. If you book your safari from late July through to the end of September then it is necessary to book well in advance. This time of year is madness. If ever you hear stories of overcrowding in the Serenegeti the chances are the safari took place between these months; and most probably the safari was in the Seronera Valley in the central Serengeti. This area of the Serengeti has a good reliable concentration of animals year round.
There are several companies that make the tours all around, prices may vary according to the length of the tour. Normally they are one-day tours, but special night tours or three-day tours are also available. There is one safari tour that will take you for 10 - 21 days from the Eastern to the Southern parts. To make the experience really complete it is recommended that you you avail yourself of a good local tour operator or a specialist. You can find a whole lot of this information out from your travel agent.