Of these two, the best on choosing a topic is Falk & Fajans, Scholarly Writing for Law Students.
Talk to your professors, or to any others member of the USF law faculty who teach or research in areas that might interest you. The USF Law Review Handbook has a list matching professors with topic areas.
Ask lawyers in law firms, government agencies, or public-interest and advocacy organizations what important issues remain open that they'd research if only they had the time. Attending events sponsored by the USF law school's Office of Career Planning is one way to meet lawyers who specialize in interesting areas of the law.