If you don't have git on machine, (install it)[https://help.github.com/articles/set-up-git/]
Fork this repo by clicking on the fork button
Now clone this repo to your machine. Click on clone button and then copy to clipboard icon
Open a terminal and run
git clone https://github.com/this-is-you/first-contributions.git
Where the url can be pasted from clipboard
Go in to that directory
cd first-contributions
Now cut a branch using git checkout command
git checkout -b add-your-name
Now open Contributors.md file in a text editor and add your name to it, save the file
If you go to the project directory and do git status, you'll see there are changes
Add those chanage using git add
git add Contributors.md
Now commit those changes using git commit
git commit -m "Add your-name to Contributors list"
Push your changes using git push
git push origin add-your-name
If you go to your repo on github, you'll see a button to open a pull request. click on that button
Now submit the pull request