Karla: If you're going to offer bookkeeping services, QuickBooks is by far the most common program in the U.S. It will be the program that your clients are using. Even if your client hires you to do "everything" and doesn't care what program you use, their accountant might want the file at year-end for tax purposes and they will readily accept a QB file.
If you're offering online bookkeeping services, where your clients can login and enter a check they wrote or run their own reports, there's lots of choices: QB Online, Outright.com, Freshbooks.com, ClarityAccounting.com etc... But many of these are extremely limited and I wouldn't recommend Outright if you're providing professional bookkeeping services ... it doesn't allow for reconciliations among many other limitations. QB Online allows you to do a lot, but isn't cheap
Deb