I don't claim that those programs will do well on your site, but they do work well on a site aimed at webmasters interested in making money. Totally different programs may work best on your site. The first step is inspecting the site to see if it's the sort of company with which you want to be associated. Then read the contract carefully. For example, some bookshops or wine sellers have an exclusivity clause - they want to be the only company of that sort on your site.
Some companies automatically accept all applicants.
Some inspect your site and advise you within a few days whether it has been approved. Next, you are given linking instructions. Some, like Network Solutions, provide the HTML code ready for you to paste into your page. Others provide you with a general piece of code and give you instructions on how to insert your special ID into the code. Network Solutions gives you no choice - if you want to be an associate, you put a graphic on your site.