To get an API key sign into your account and open the Account Options page, Account Details tab.
Below your account id, you will find the Get Api Keys link which will generate your API key and secret key.
The API key and secret key are generated for you alone. You should make sure that they remain secret and others do not have access to them. If someone finds out your API key and secret key there is a Reset Api Keys link which will generate you a new set of keys.