Basically you either start your own blog, website or sign up to HubPages - the easiest place to start. You also need to join an affiliate company such as Amazon or eBay. Then you advertise items that are sold on say Amazon and if anyone clicks on your link and buys the product (or anything in Amazon's case) you receive a percentage of the sale.
For example you could join the Amazon affiliate network and choose a book to write a review about. You then start a hub on here, and include an Amazon affiliate box. You place the link to the book you are reviewing in the Amazon box and then publish your hub.
Then if anyone reads what you have written and decides to click on your link to Amazon and buy anything from the site, you will receive a percentage of whatever your reader buys. I hope this helps.