Most Ebook Central sites require you to sign in to gain access to the site – and typically that also signs you into your personal account (also referred to as your bookshelf account).

You’re all set if it says “Sign Out” at the top right -- that means you're already signed in to your personal account.
User-added image

If not, click the "Sign In" link or wait until you're prompted to sign in.

If you're browsing without first signing in (some Ebook Central sites allow this), depending on your library's settings you may be prompted to sign in once you attempt to: You will always be prompted to sign in when you attempt to:
If you don't know your login information, contact your library staff.

Once you are signed in, you can review your personal account settings and make changes under Settings.