Yes you are very right niched really matters alot because a trendy niched makes alot of money another factor is traffic you need a good traffic to make some sales effective promotion is the key to success in affiliate marketingThe guide for Affiliate Marketing is simple and straightforward, I think first thing to do is to decide on a niche that would be way for you.
After deciding on a niche, Find for a platform you intend to use for your business.