When starting a new business, one of the most important decisions you have to make is who exactly you're going to target with your marketing efforts.

Assuming you're not equipped with millions of dollars of venture capital funding, it's probably not a good idea to try and compete on a huge scale with the Amazons of the world. Much better to try and focus on one niche, and try to really dominate that market.

A First Attempt (Still Learning...)
My first business that I started was a publishing company that publishes books about sole proprietor taxes and LLC taxes.

The business has done decently in terms of profit (mostly thanks to Amazon's truckloads of traffic). However, it certainly hasn't reached anything like a market-leading position.

Second Try (A Little Closer...)
My second business was/is an online training course that teaches people How to Make Joomla Websites. Obviously, given the focus on a specific piece of software, it's going after a more narrow market than the entire small business industry in the U.S.

As a result, this site has been able to reach page 1 for several fairly short-tail search terms in its niche. Despite being over a year newer, it has obtained nearly the same level of traffic that my tax site has. Getting closer!

This One Looks Good
A friend of mine has recently begun a site that plans to focus entirely upon customer reviews of high-end cutlery sets. (His idea is to gain traffic, then monetize via affiliate links to Amazon or elsewhere.) Now that seems like a pretty targeted niche.

He's still building the site, so we'll have to wait and see how it turns out. However, it wouldn't surprise me to see him dominating many related keyword searches in a comparatively short period of time.

Holy Cow! This Guy Really Has a Niche
Through a family member, I recently heard about a gentleman in Wisconsin who runs a recruiting business/website all about Physical Therapy Jobs in Wisconsin.

Given that I don't know him personally, I can't make any statements as to his profitability. But if you do a few test searches, you'll see that he really seems to run the show in the search results for his market. And given what I've heard about headhunting payouts, I bet he's doing just fine.

Takeaways
My suggestion? Take a look at your interests, and find a topic that you'd be interested in writing about/creating videos about/etc. Then brainstorm as to how you could narrow it down until you'd be one of only a few players in that market.

When you're in a sufficiently-defined market, you'll start to see results coming pretty quickly in terms of traffic. And it doesn't take too long to start turning visitors into customers.

Best of luck with your new businesses!