May I suggest checking out our Instructor Quick Start guide? https://help.turnitin.com/feedback-studio/turnitin-website/instructor/quickstart.htm
It will guide you through the initial steps needed to get started with Turnitin, including how to add your first class.
We have more substantial guidance, covering pretty much anything you'd like to do with Turnitin available here: https://help.turnitin.com/feedback-studio/turnitin-website/instructor/instructor-category.htm
I've added a handy video below that will also guide you through the process.
I hope that helps you, but please do feel free to reach out if you are still unsure :)