When figuring out what niche your website should be in you have to ask some questions to yourself.
1.Am I passionate about the subject?
2.Does it sell or make money (unless your not worried about that)?
3.Are there products being sold or affiliate products available to sell?
4. Is it getting a lot of searches?
To find out if what you want your website to be about is making money do a Google search for that topic. Look to the right to see if there are any Google ads. If there are, it's likely it makes money, cause no one is going to pay for Google ads when something isn't making money.
Also go to Ebay and Amazon to get some ideas and see if it's something popular.
The surefire way would be to go with something you already know is popular, but remember these are also going to be highly competitive. Like weight loss, finance, making money, electronics etc.