Competitive niches are competitive for one reason. They have a huge amounts of people who are buying stuff.
With a lot of buyers comes a lot of sellers and that makes a niche very competitive. And that is the exact place you want to be.
Niches like Fitness, Dating, Biz Opp are all there because of huge amounts of buyers. But how can you jump in and be competitive with others trying to sell?
Find a good angle and run with it. The only reason Facebook is as big today in the Social Media niche is because they found the faults of Myspace and a different angle and ran with it.
Google is in the Search Engine niche. The reason they are huge is they found the faults in Yahoo put there own angle and ran with it.
To be competitive you need to figure out how you can jump in with something different. Or jump in a improve on something else. It does not have to be major, just a small change to improve on a concept could make you a fortune.
Ok, now what ever you do. Do Not Jump into a niche that is not competitive. There is no money in it. How many buyers do you think there are in the Gardening NIche? Not that many, so this make is hard to make any money.
The bigger the competitive niche, the bigger your fortune will be after you find how you can be different and stand out.