You have to first decide on the type of business you want to start.
Then do research about that type of business.� Organizations like the Small Business Administration and SCORE will help you.
It's also good to take a look at what other businesses of the same type are doing online. How are they marketing their businesses? How do they get new customers?
Once you've got all the background you need, read a little about what it takes to start an online business. "Starting an Online Business for Dummies" by Greg Holden is a good start.
Lena West is the CEO and Chief Strategist of xynoMedia Technology, a New York-based firm that helps high-growth companies leverage the power of social media, blogs, podcasts and online communities.