You must have a GitHub account to fork an exisitng repo. If you do not have an account create one by following these instructions Signing up for a new GitHub Account
Make sure you are logged into your GitHub account
This example show you how to fork the quickstart-linux-bastion repo.
Using your web browser navigate to the aws-quickstart/quickstart-linux-bastion repository.
Click Fork button in the top-right corner of the page.
Choose the account you would like to use for this fork.
You now, have a fork of the original aws-quickstart/quickstart-linux-bastion in your selected GitHub account