I'm sure that there are plenty of useful apps, but at the top of my list would be WorldRemit.com, MTN Mobile Money, and Google Maps. You may have a hard time getting the MTN Mobile Money and so you might need the help of an established local with that. You will need WorldRemit.com to get money from your bank account back home into your new Mobile Money account. If you can't get an MTN Mobile Money account, you'll need World Remit or MoneyGram or Western Union to get cash from your bank back home. All have mobile apps.
Ghana doesn't have home addresses like we take for granted in the United States. And Ghana doesn't use plastic for everything like we do here. Some do take credit and debit cards, but for the most part, Ghana is a cash economy. You can use MTN Mobile Money to store cash and have it available without actually having pockets-full of paper money.
You will need Google Maps for obvious reasons, just to know where you are and how to get where you want to go.