You must understand that the Merchants do not pay the commissions to affiliates directly, they associate with some third party processors to take care of you, a very good example of such third party is "clickbank.com". The Affiliates are also referred as "Publishers" in some places.

The third party processor acts like an interface between the affiliates and the merchants. They provide you the host of products which you can choose and do the sales promotion and generate commission. So you need to register with them first and get a username and password. You must record them safely as you have to login and check how much income you have generated from each promotional activity. The Registration process is "FREE" and simple. Once you are through you can choose the Merchant, his product and required sales promotional links with your tracking code embedded from the third party itself without even going to the Merchant's site.