Almost all affiliate marketing programs are completely free to join. Some might even award you a sign-on bonus. It does not cost companies anything to start an affiliate program so be wary of any that charge you. You might spend some money upfront on a website service and domain name, however.
Affiliate marketing essentially makes you a salesperson for another company, with no risk involved! You receive a personalized link from a merchant, and when someone clicks on that link and makes a purchase from the merchant, you will receive a percentage of the sale value, or a flat rate, as a sales commission. Affiliate marketing works best if you have a blog or social media platform where you can attract an audience.