If you've signed up to Kyso with Github you will already be connected and you should see all your public repos. If not, just click the Connect Github account button. Using the dropdown on the right you have the option to authorise all your Github repositories, or just your public ones (default). Its not possible to authorise just one repo at this time given the way Github's OAuth authorisation works.
On the Github OAuth page you will have the option to authorise your own account and any organisations you may be part of.
You can search and filter your repositories, and import them to Kyso by clicking Connect to Kyso. You can then browse to your post on Kyso and choose whichever file you want to be shown to your visitors by default.