How to get a niche in affiliate marketing?:

How to get a niche in affiliate marketing?

For example, many people are promoting weight loss products but where do they get the contents to write about? Do they actually buy the products first to test it out themselves before pitching for it?

I have interests in a wide variety of things but I don’t know about a specific thing well enough to write a whole blog about it. How is it possible to start this way?

What the first steps to starting affiliate marketing? Is it finding a niche, getting a web hosting site for a blog, then blogging about the products? What’s google adsense and what does bidding for words mean?