Once registered, login to the account and go to Tools in the main menu.
In the Tools area, find the heading Developer Program, below this heading is an option that will say Web service and APIs. Click this option.
Within Web service and APIs you will see an option to sign up for the Developer's Program. This is a free service. Sign up to this service and follow the prompts.
Once signed up for the Developer's Program, the next page should show a box with the heading API Keys. Within this box will be 2 rows, one row will say Development and the second row will say Production. You want the username and password in the Production row.
Once you have these credentials, navigate back to your website's shipping settings and add these credentials into your Canada Post shipping method and save.