When you signed up with us, we automatically created an "Application" for you in our Accounts Management Software, which controls your API Key.
To find your API key, simply click the Get your API Key link in the left column of the Accounts System. You will be taken to a page which shows you details about your application, including showing you your API Key.
Make a note of this API Key and keep it secret. It controls your access to the API. Anyone with a copy of it can access the API as you and use up your monthly quota.
Read the section on Authentication to see how to send this API key with your requests.
If your API Key is compromised, other parties will be able to send API requests on your behalf. If this happens, you need to generate a new API Key and stop using the old one.
To do that, click the big blue button labeled "Create New API Key". When you have done this, you will see a new API key, and the old one will stop working.