To overcome this problem it is best to look at your own interests, because you will have knowledge on these subjects and obviously will enjoy working in that area.
How do you know if your hobby is in a a buyers market?
Well I think this is a good method. Take a trip to your local newsagent, the type that have several shelves of magazines. If a publisher invests time and money into publishing a regular magazine that means their is a market for it. They don't normally waste their cash.
Plus if you are looking for content then the magazine will be full of it and this should spark off a ton of ideas. Also it will be a great place to see what problems people are having in your chosen niche.
Making money online can be as simple as finding problems and solving them, and this is a great method to get you started.
Of course there are many other things I could add but this is just a short post with a basic idea of how to choose a niche. I hope it helps someone.