When it comes to starting an online business, the first thing you should consider is what you already know. Make a list of your interests, hobbies, education, and employment history. At this time, don't make any changes to your list. Include anything you can think of that you do or enjoy.
Take advantage of this opportunity to think outside the box. Remember those classes in college, high school, and continuing education where you learned something that could be useful to others? Keep track of the books and publications you've read (and the information you have gleaned from them). This is the time to write down whatever knowledge you have rattling around in your head as strong points.