Well it depends on the type of website. You could have a contract with YouTube and have to make a certain amount of videos a week. And if you do you'll get paid a good amount of money. Some other sites want thou to participate with them such as advertising their logo on your car or on the web.
If you own the website, then it would be best to join an affiliate program. This program will enable you to sell products belonging to third parties and earn a commission. Alternatively, you can just start selling stuff belonging to you on the website.