To set up an API key, first visit the API documentation page.
On the page linked above, click on the first link, Request a new API key, or click on Manage Applications in the upper right of the toolbar.
Next you'll navigate to the application link, here. Follow the steps to apply for your own API key.
Remember that each individual application or use of the API should use its own key.
After completing the fields in the form above, you'll click Apply and review confirmation that the API key was issued.
To manage your existing API keys, visit the API Documentation page and click on Manage Applications in the upper-right corner.
Click on Manage Applications to make any changes available to your existing API keys.
Here you can update an existing API key, or even create a new key.
After you select an existing key, you'll preview the details about that API key and have access to edit the Name, Application URL, and Callback URL as needed.