There are ATMs in Osu at gas stations and some restaurants. If you are going to exchange money or take out money, get a van to take you there and back so you are safe
International VISA card: do not leave America without it! Mastercard is not widely accepted so you may need to switch banks to ensure you have access to your checking and savings accounts.
These can be exchanged at the bank at Legon, a bank in Osu, or a resort like LaBadi. However, in general they are hard to cash and the exchange rate is terrible. The benefit of using them is if they are stolen you can be reimbursed.
You can convert your money to Cedis at Forex Bureaus located all over Osu with one close to the houses in the same plaza as the Coffee Shop. A new Cedi (New Ghana Cedi or NGH Cedi) was recently introduced making the exchange rate almost an even $1USD to one Cedi. In addition to paper money, there are coins called pesos in 5, 10, 20, and 50.
Some people still use the old money and taxi drivers will sometimes still talk about pricing in the old currency, so it is important to have an understanding of the old system as well.