I'm not very successful in terms of affiliate marketing, but I would say that it is best to start out on a niche that you really know. Yes, you've probably heard about this several times already but it is worth repeating. Review a product that you have used yourself, and provide your readers with an honest review. Other than that, I highly suggest you learn about "buyer keywords".
The thing about affiliate marketing is that you need to have a great website first, and one which gets a lot of traffic. You can achieve this through
SEO and various marketing methods. So if you want to be successful in affiliate marketing, you have to learn the other aspects of Internet marketing as well.