Make sure you bring some cash, but not too much. Do not carry amounts in your pocket that you would feel uncomfortable with at your local supermarket, and try to keep your money out of sight and safely tucked away.
There are a some Barclays Bank branches where you can withdraw cash from the ATM, there are a few in Arusha, and another branch in Zanzibar (Stone Town).
The official currency here is Tanzanian Shillings, however US dollars are widely accepted, though preferably not the old 2003 or 2006 notes. You can use your credit card in the large hotels and lodges (to pay for your drinks for example). You won’t be able to use your card in remote places in the bush. There are many bureaus where you can exchange money in Arusha