You need to do this only once. The instructions here are very similar to the instructions at http://help.github.com/forking/ — please see that page for more detail. We’re repeating some of it here just to give the specifics for the Sage Notebook project, and to suggest some default names.
If you don’t have a github account, go to the github page, and make one.
You then need to configure your account to allow write access — see the Generating SSH keys help on github help.