Join a affiliate program and then you'll given an affiliate link which you can start promote it. You will get paid only when you make a sale, the payout is usually once a month or once every 2 weeks in case of ClickBank.
Let me put it simple! # you promote others products and sell them and you will a commission for that # There are lot of sites where you can signup and become an affiliate and promote others products(eg:clickbank.com) # Simple as this, you drive some traffic to your affiliate sales page, people buy the product through your affiliate link and you get paid. # It involves a lot of SEO to be done so, beginners have to work a lot and do a lot of research Good luck