Fork this repository by clicking on the fork button on the top of this page. This will create a copy of this repository in your account.
Now clone the forked repository to your machine. Go to your GitHub account, open the forked repository, click on the clone button and then click the copy to clipboard icon.
Open a terminal and run the following git command:
git clone "url you just copied"
where "url you just copied" (without the quote marks) is the url to this repository (your fork of this project). See the previous steps to obtain the url.
For example:
git clone https://github.com/this-is-you/first-contributions.git
where this-is-you is your GitHub username. Here you're copying the contents of the first-contributions repository in GitHub to your computer.
Change to the repository directory on your computer (if you are not already there):
cd first-contributions
Now create a branch using the git checkout command:
git checkout -b <add-your-new-branch-name>
For example:
git checkout -b add-alonzo-church
(The name of the branch does not need to have the word add in it, but it's a reasonable thing to include because the purpose of this branch is to add your name to a list.)
Push your changes using the command git push:
git push origin <add-your-branch-name>
replacing <add-your-branch-name> with the name of the branch you created earlier.
If you go to your repository on GitHub, you'll see a Compare & pull request button. Click on that button.
Now submit the pull request.