Always disclose your affiliate relationship. Most visitors will probably understand that graphic ad will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.
If you’ve got a way with words and expertise in a niche, there are plenty of sites that will pay for articles and content you write. Think of the sites you read regularly. What can you contribute to them that would be interesting? Research your niche and then look for ways to pitch articles. Many sites will simply have a submission or contact link in the footer. To get started, check out my full guide to becoming a freelance writer on the side and then submit your articles to places like Instash, Listverse, A List Apart, International Living, FundsforWriters, and Textbroker.
Once you have the affiliate links you need, it is time to present them to your audience. As you start using affiliate marketing to talk about products, you need to be aware of how your audience perceives you. If your website appears to be only after their money through an overwhelming amount of affiliate links, then they are more likely to simply leave your site.