There are different kinds of affiliate marketing. There are digital products sold on sites like clickbank and jvzoo. You have CPA which stands for “Cost per Action” which means the referrer gets paid when someone does something, the simplest form of cpa is email submit. An email submit is where you send someone to a web form and when they submit their email, you get a commission.
There is also Adsense. This is where you host google ads on your site and get paid when someone clicks on them. Some people build niche sites that are specifically designed for the adsense program. These sites can be lucrative in two ways. Number one, if done right can generate a quite sizable monthly income, and number two, these sites, once established, can be sold for quite a bit of money. I am not really sure if this is affiliate marketing or not but many feel it falls into that category.
There are dating sites as well as membership sites that will pay commissions. If you do not mind the subject matter, quite a bit of money can be made with gambling and porn sites.
And the biggest of them all is the Amazon associates program. There are people making 5 figures a month as a amazon associate. Most retail sites, like Walmart, Target, LL Bean, the list goes on, all have affiliate programs. When you get to their page just scroll down to the bottom and look for a link to affiliate program, associate program, webmasters, or anything similar and you will see what you need to sign up.
The reason so many go with Amazon is they can convert sales like no one else, and you get a commission once the customer is there, no matter what they buy. You might, for example, have a link on your site promoting the X-Box 360, but the customer ends up buying shampoo. Guess What? You get the commission.
So, do a google search for “affiliate networks” sign up for a few, it costs nothing, and see what is available for you to promote and make money.