You can obtain API key for your account here.
Remember, API keys grant rights to modify (and destroy) data on your account. You need to keep them in secret, and if some key leaked, you need to revoke this key, and generate new one (and tell your applications to use the new key, or they will stop working).
The Wordpress key can only view, add, update and delete placeholders whose name starts with "Wordpress: ". There is also Full-featured API key.