1) Fork a project to your own github account:

2) Clone the project which in your account to local:

git clone https://github.com/this-is-you/first-contributions.git

3) Create a new branch, and switch to the new one.

git branch -b my-contribution

4) Do your job!
5) Commit your changes into your local repo:

git commit -m "update changes"

6) Push your commit to github:

git push origin my-contribution

7) Pull Request to original project: