If you have registered for an account on WordPress.com, you will also have a Gravatar URL reserved for you based on your WordPress.com username. You can create your Gravatar account by uploading a photo to your WordPress.com profile or by logging in to Gravatar at any time using your WordPress.com credentials. If you don’t yet have a WordPress.com account, you’ll need to sign up for one to use Gravatar.

Have a site that runs WordPress, but isn’t hosted on WordPress.com?

The WordPress platform is free, open source software that you can install on any server. It does not share account information with WordPress.com.

It’s possible for you to have an account on your own WordPress installation but not on WordPress.com/Gravatar. An easy way to tell if you have a WordPress.com account is to try and reset your password using your email address instead of a username. If you have a WordPress.com account, you will receive a confirmation email and can proceed to use that account.