Before you can use GitHub to track changes to your code, you need to create an account.
Go to https://github.com/join to do this.
If you're setting up GitHub for a school project, you might want to consider using your school e-mail address so you get notifications there. Otherwise, just make sure you use an e-mail address you check regularly.
Your username can be anything you wish.
If you run into problems, GitHub has help documentation on setting up a new account.
GitHub offers several plans.
For most purposes, the free GitHub account is more than sufficient. The "pro" option allows for more collaborators in private repositories, and provides additional options like GitHub Pages, a Wiki, the ability to protect branches of your code, and access to repository analytics.