You just need to create a GitHub account, create a public repository on the GitHub website, and add your Unreal project folder. Every time you make significant changes you want to update those changes to GitHub by going to the website and doing a Commit in your repository. The rest of us can then access your project by going to https://github.com/your-account-name/your-repository-name.
Hope that helps. Maybe Unreal Engine also has a Git plugin, but I don't know.
