When you join a web site affiliate program, you enter into a partnership with the associated merchants: the merchants in the program each wants to sell its products and you become their online sales representative.
Most web site affiliate programs provide you with the HTML code for the ads that will go on your site, as well as banner graphics and marketing ideas. Furthermore, many web site affiliate programs will help you determine where to place these ads so that they have the greatest possible impact. Make sure the ads you use are tasteful and fit within the parameters of your web site. Most affiliate programs offer ads in a variety of sizes, shapes, and colors, so you can find one that works for you.
Read the materials you are given when you sign up for an affiliate program and try out their ideas. If you don't have a background in marketing, you may be surprised at what methods of online advertising work to boost your income.