For instructors: your Turnitin account is automatically created when you create a new Turnitin assignment on Moodle. If you want to use your account to login to Turnitin.com directly, you will choose the "forgot my password" option on the Turnitin.com website and it will prompt you to reset your password.
For more information about using Turnitin with Moodle view the Turnitin Instructor Manual and Student Manual.