On the top right corner of the repository page you will have an option fork
Follow the commands given below
git clone <link of the repo>
cd <repo name>
git branch <branch name>
git checkout <branch name>
Follow the commands below
vi <your file name>
Type your code
git add .
git commit -m <message>
git status
git push -u origin <branch name>
Click on the new popup compare & pull request
Give a title and leave a comment
Click on Create pull request