GitHub allows you to create your own git repository in their server free of cost, and you can publish this publicly at their website github.com. SO basically GitHub is an free hosting platform for your git repositories which you can share with anyone to download/clone or other people can contribute in a distributed development environment.
To create your git repository on github, first thing is you should have an account created on their website. If you have not registered, visit https://www.github.com and click on “Sign Up” fill the form and complete registration. After registration Login to your github account from https://github.com/login
After login, from top right corner click on + icon and click “New Repository”
After this, you will be redirected to url, which shows a form to fill where you would need to mention repository name, description, whether to make this repository public or not and add a default README. Complete the form and click “Create repository”