That’s pretty easy.
Register.
Lol...
Joking.
Pull is your best friend to keep up with the updates from the main (online) repository.
There are more basic options, but Commit (to your Mac repository/directory) and Push (upload your Committed changes) will be the most common usage.
But you should have been registered to github first before it.
No Joking.
@labasus,
You might want to just initiate the github-ing.