Here's a great lesson I've had over the past years building things: Finding a niche is not enough.
I previously started StudentFlatMaastricht (an online letting for international students that grew to 250k/year) and now run ManyPixels ($50k/month)
Starting with a niche is important (so that you can build a product/service tailored to the needs of that niche, and it's easier to find ways to market to it). But I would go one step further: Focus on users that have the "highest urgency" to buy your product. Or in other words: Focus on the "white-hot center", users that have a burning pain to use your product (in the words of James Currier).
For example, for my online letting agency, I started only by focusing on international students (my niche), and my "white hot center" was students from Singapore since they couldn't come and see the place in person, and I catered solely to them at the beginning by recording videos, getting on Skype calls with them etc. It gave them HUGE value and kickstarted things from there.
Same for my design-as-a-service. I started with users that had a very very urgent need of making illustrations for their landing pages.
TL:DR: Find a niche, and find the "white-hot center" in that niche.