Picking a niche is one of the easiest parts of making money online, yet many people tend to struggle with it (they over think it and over complicate it and make it more difficult than it needs to be).
This is a strategy I use to help pick a niche that will likely return good results, it's certainly not the only way or the "best" way (I don't believe there is any "best" way).
If you're stuck and can't think of a niche, look for one where you can legitimately help people solve a problem. In affiliate marketing, the type of offers that do well are the ones where people are looking for help (eg. diet offers - people are looking to lose weight, loan offers - people need money). While I am not saying these type of affiliate offers actually help people (maybe they do, maybe they don't), they target people who NEED help.
If you pick a niche that helps people, where people are looking for help, it will help you.