1. own a website.
2. Go to the website of who you want to affiliate with. Amazon, Staples, etc. They will have an associate/affiliate sign up link.
You can also go to major affiliate sites like linkshare, reporting.net, cj.com, from there you can sign up for many websites.
3. they will provide you a link.
4. place the link on your website..
5. start making money
Well you would go to - say amazon.com, & become an affiliate. They would then give you a code to insert into your web page. The only time you got paid would be if someone clicked onto that banner from your site & bought something from Amazon. They would not pay you just to post their banner. This is advertising.