To see Etosha you need to drive around the park. There is no way to walk within it, or to fly just above it. If you do not have your own vehicle then you must either hire one, or book an organised trip.
Hiring your own vehicle is best done in Windhoek. However, if you are just travelling though, and hiring a car just for Etosha, then consider doing so from Tsumeb. This is normally best organised in advance, through a tour operator or the car-hire companies in Windhoek some of which will let you pick up and drop off cars at Mokuti or Ongava.
Organised trips to the park emanate from the private lodges around the park. Other than these, many operators organise guided trips around Namibia including a few days in Etosha, often staying in the National Park's accommodation. However, you only need to see one air-conditioned 75-seater coach driving through the park to convince you that this is not the best way to visit either Etosha or Namibia.