Generally, startups are formed by people who have an idea that is from their background, and which has a good chance of succeeding in the market. So you should look to your own interests and training background for ideas, and then look at the potential markets for those ideas.
If you are still in training (like still in school), you can look at potential growth markets, and tune your education and initial work areas towards those promising areas. For example, biomedical engineering has some very promising areas for new products, and homeland security will grow a lot over the next decade or so.
What areas of study or industry interest you the most?