Come on now. You really just asked that? You couldn't do any research on your own? Trust me - if you aren't smart/self-suficient enough to find out how to "start earn by Clickbank" - you wont be able to make money anyway.
All affiliate programs work by affilite codes which are put in urls. So for every clickbank product you will be given a specific URL and you will put your "code" or username in the appropriate place ( basically xxxx) in the URL then promote that specific link. You don't even need all this bcause there is an automated wizard that will help you do that. You can get a lot of stff within clickbank itself.
@dlm is right. If you can't start of your own, you can never earn money with Affiliate Marketing. Ask others for tips to improve, not how to start with.....
for beginner with clickbank, it is probably easier to start by selling other people products first unless you have a good offline product that can be converted into digital product and you are willing to do that.