Short answer- find a market that has low-competition/high-demand/with weak competitors and has commercial value, aka the "money keyword", thankfully there are tools to find these.

When going for an e-Biz, it's all about starting with a good foundation and knowing how revenue is achieved:

For some it's an e-Store with drop-shipping, for others it's affiliate marketing and then there's the info product sellers(who usually depend on affiliates to help build their business).

What they all share in common is- they need to have a market defined as "a group of people with the ability/intention to pay", markets are usually defined online through search keywords, i.e. "underwater kick boxing". Upon finding this you need to drive traffic to your site, there is either organic which costs little to nothing money-wise, but a lot time-wise, where you build the quality of your website to rank highly for your targeted keywords and build traffic OR paid, which is usually through Google Adwords or the newer Facebook Demographic PPC, both of which involve more money and less time, but still a good amount of it in addition to focus to be profitable. Next up is building a loyal community/converting this traffic into sales, usually through a drop-shipping product OR affiliate offer. Then the final step is finding what the people want and will pay for and having your product to sell which means welcome to the Fastlane