Commission Junction is an Affiliate Network... Make money by choosing a good converting product and driving targeted traffic to it.
you have to choose a company who's products you wish to promote and then promote those products either on your web site or with pay per click advertising etc, use your affiliate links from the company and if someone buys their product or service via your link you will earn commission. you need to join cj first, which is free, then apply to an advertiser, make sure that you read and understand their terms especially regarding ppc bidding and so on. good luck
Make sure you choose merchants carefully. Choose merchants that have a high CPC not just for the past 7 days but also the past 3 months. You can also see if they are one of the bigger merchants by looking at the green bars, the more the better. Also search for them on this forum to see what others are saying about them. I'm just saying this because there are so many different merchants and a lot of them don't convert at all and you don't want to waste your time with them.