There is no charge to join an affiliate program. However, some promotion strategies will cost if you choose to use them. For example, while your blog won’t cost you anything but your time, pay-per-click marketing, email campaigns, and social media ads will require an investment.
Affiliate Networks act as a middleman between companies and their affiliates. They manage the relationship and providing third-party checks and balances.

It’s possible to do keyword research using a free tool such as Ubersuggest, but better and more accurate results come from paid-for tools. The guys at Authority Hacker have written an excellent post that runs through the options.
Some topics are naturally so big that you can always extend them with content and marketing. However, as a beginner, you might not be able to work on big projects. Choose a niche instead that is expandable. Let’s clarify the topic with an example. When you’re choosing fitness, women fitness, child health — these are big niches, and you can create plenty of content around them. Likewise, if you go for “Yoga outfits”, now that’s a microscopic niche.