You don't need to register on the website to join the server. When the server asks you register you need to make a password for your account. The command for that is /register password password (replace password with an actual password). After you register you can login with /login password. To change your password you'd use /changepassword newpassword oldpassword.
If the server doesn't ask you to register but tells you to login on your first join, that means someone has already registered your account name. If you're using a cracked launcher you can change your name and see if that account name is not registered.