The official currency is the Tanzanian Shilling. You can exchange at most major banks in the country. Also, you can use your credit card (Visa or Mastercard) to withdraw money from ATMs. Many find this to be the most convenient way to get Tanzanian shillings.
However, US dollars are required for many things such as park entry and visas. (Your park entry is included with this trip, so this is more for your FYI if you plan to spend extra time in Tanzania and visit other parks including the Serengeti.)
US dollars are also accepted in most tourist areas. It is highly recommended that you bring travellers’ cheques in US dollars and a small amount of US cash.