The products and services you will be promoting to your audience must be relevant and good quality. Make sure you believe in them and know everything about them, because this will be crucial to you delivering the sales pitch to your audience. You need to build trust with your audience so make sure the products and services you choose to promote are trustworthy enough.
One of the reasons that it's so successful is because the program is free and extremely simple to join. The interface is also easy to use. This means that you can start earning money right away once you've signed up. You earn money when a customer clicks on one of the links you're promoting and then buys something on the site. It doesn't even necessarily need to be the product you've recommended.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads 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.