Both an access key id and a private (secret) key are necessary in order to access the Amazon Web Services Product Advertising API. This is the process for obtaining one:-

1. You need to create an account at http://aws.amazon.com - this is free.
2. Once you have created your account, you sign in at https://console.aws.amazon.com/ec2/home . Ignore the message to sign up to EC2, you don't need it.
3. Click on account->security credentials
4. Look down the page to 'Access Credentials'. Your access key id is displayed in the table under 'access key id'. Your private (secret) key can be displayed by clicking the 'show' link next to it.

As its name suggests, you should keep the secret key to yourself. This is used to digitally sign the requests that you make to the Amazon Web Server.