To start affillate program to their site and complete online registration, to get list of affiliate program and network visit the link. Some program approved your online registration instantly some others require a human to check out your application before it is approved. Their (site) will give you some HTML code. which you can cut and paste into your web page. Note that some affiliate networks and programs will not accept you unless you have your own domain name. If you are planning to earn from your site, you should seriously consider registering your own domain name.
Do not choose those with payment schema. Do not paid for registration in affiliate program. if you are targeting parents on your site, links to affiliates with educational software, books and the like may generate more revenue than banners that link to web hosting companies. The most important rule of choosing an affiliate program is to know your target audience. if your site has a particular theme, and an affiliate network only supports one or two suitable advertisers, you might want to sign up for a few affiliate networks so as to get a greater number of relevant advertisers. After all, advertisements that are relevant to your audience are more likely to be taken up than general advertisements. (What's the point of putting banners from only one affiliate if nobody is going to click them?)