The first step is to create a Commission Junction account, if you do not already have one.
Once you have an account, login and click on Account >> Websites.
From there you can add your website or grab one of your current Campaign IDs:
Next, we need to get your developer key.
To get your developer key you will need to login to the Amazon Web Services (using your CJ username and password).
Follow the instructions and a developer key will be emailed to your and also shown on the website: