The first thing you'll need to do is connect your Github Account to Zapier. After you sign up and start creating the Zap you want, Zapier will ask you to start the process:
You can then give your Account a title or label, which is just for your convenience and helps you track multiple accounts. It doesn't have any affect on which Account you actually add to Zapier.
The Github account that you are logged into will be displayed next asking you to confirm Zapier's access:
You'll also be prompted for your password to continue.
Finally, you'll be redirected back to Zapier and the Account will have been added and tested, if everything is good to go you'll see this:
Now you can continue creating your Zap.