Once you've decided to be an affiliate marketer, it's important to choose a niche you want to work in.
By focusing on one market, overtime, you can begin to dominate it.
It makes sense to target fitness, for example, and write articles that rank well for fitness, instead of writing for a large gamut of topics and doing well in none.
Figuring out what niche you want to dominate depends on two factors: Saturation and Passion.
Market saturation determines how hard you'll have to work to dominate your niche.
The more saturated or more people actively attempting to market your niche, the harder you'll have to work, since theres more competition.
You can find out how saturated a niche is by searching for a product in Google.
If there are a lot of ads, or multiple websites promoting the same deal, the market may be saturated.
Another tip is to use a backlink checker to determine the number of backlinks the top ranked site has.
This gives you an idea of what you have to work towards.
The second factor, is Passion.
This has little to do with the competition but is something that you can control.
If you're extremely passionate about a subject and can write for hours on the same topic daily, then this is the niche just for you.
A high level of passion can lead to success even in an extremely saturated market.
Finding the right niche takes time though.
So be prepared to test a niche and don't be afraid to scrap a project if it doesn't work.