A good way to find a niche is to work with something you already know how to do. Ask yourself what you're good at ... what are your hobbies... then work with that. That's not to say you can't enter a niche you know nothing about, it's just easier if you already have the knowledge.
You could also check on places like ebay and amazon to see what the best selling products are. This is what people are buying and willing to spend their money on, and should show you a profitable niche.
Then you need to find good keywords.
The key to good keywords is to find those that have low competition and high search volume.
Once you find some keywords with a high search volume, plug them into google with quotation marks.. i.e. if my keywords are make money online, plug in "make money online" into google - this allows you to see how much competition you have for that particular phrase.
Anything with a search volume of 500+ and competition of 10,000 or less is a good start. It goes without saying though, that the higher the search volume and the lower the competition is, the better.
A lot of people though, make the mistake of thinking that 'saturated' niches can never make you money... you can make it any niche if you put your mind to it
If people are rushing to that niche, there must be money in it! New restaurants pop up all the time, yet there can be hundreds of restaurants already operating in a particular city ... it literally all depends on what they bring to the table