Depends on what type of affiliate marketing you are doing and whether or not you want to be able to track it. If you want to track it you more than likely need a service. A lot of website hosting providers offer services. Yahoo! and Godaddy have links available on the control panels for your website. I would start there and of course the service providers will have tips and instructions available for you there. I used Yahoo!’s trackable links program for our affiliate marketing.