In the past I used to tell people don't fork the code, but on github that is exactly what you should do. On github a fork is like a branch but in a separate repository that is your own. So you create a fork, clone your fork, commit changes to your fork, then you can submit changes that have been committed to your fork back to the original repo and you can also sync new changes from the original repo into your fork as needed.
If you go to the githib home page, they have some cartoons for github bootcamp that link to tutorials. I admit I've been using github for quite a while but only looked at those tutorials a few days ago and found them very helpful. It doesn't take long to go through all of them.