UPDATE: Tutorial updated to match the current GitHub client UI.
Basics on collaboration
You may just seen Ashley's tutorial about how to collaborate on projects with SVN. I recommend you to read this tutorial before doing so.
What is GitHub?
GitHub is a place which projects are shared with co-developers, friends and teams.
Create a GitHub account (for who doesn't have one)
Go to github.com/join and complete 3 steps to register.
1. Enter your username, password and email. After filling all boxes, click/tap "Create an account".
2. Choose your plan, and proceed. You can pick a free plan if you want to share your projects. But if you want to make your projects private, paid plans are required.
3.All done, you can now go to your GitHub dashboard when your e-mail is verified.
Sync your project from a PC
After you signed up, it's recommended to install GitHub for Windows or Mac. So you can sync your projects without opening the browser.
Preparation and setup
1. After installing the GitHub client, sign in with your GitHub account.
2. Create a repository by clicking the "+" button.
3. A balloon dialog will appear, where you can enter the name of your repository and the location of the repository. Ignore the "Git ignore" dropdown, and click/tap "Create repository".
4. Now create your C2 project, and save as project. You must create a subfolder in your repository's directory and save in there, so Construct 2 will never detect the folders and files generated by the GitHub client.
5. If you're done, you can publish your repository to the GitHub website now by clicking/tapping the "Publish Repository" button. (if you doesn't publish it yet). But first, make sure you commit the changes in the repository.
Everything is done! Now you can share the link of your repository with someone you know.
One more advice
If you make some changes to your project as your repo is in GitHub, you can sync your local project with the project in GitHub by clicking "Sync".