How involved can you be?  Some people prefer to be directly involved in the business, but for those who open a franchise on top of working a full-time job, this isn't an option.

What are you good at?  Examine your skill sets, what you've acquired in your previous work experience, and what you're comfortable with.  Are you comfortable with sales and cold calling?  Do you like interacting with customers behind the counter of a retail location?  Can you manage people?  Do you prefer instead being out in the field working with clients and customers?

While you may have the relevant skills, you don't have to be an expert in your field, just yet.  Franchising allows you to reinvent yourself and go into a completely different industry and leave the functional expertise to the franchise company.