Create a new branch using git checkout -b MyNewBranch

Start making your changes, either editing files, or creating new ones. You can use git diff to see changes between what you have locally and what the last checked in repository is.

If you added new files, you will need to use git add filename to indicate that there are new files for git to manage.

Now you commit the changes locally using git commit. This creates a new revision and drops you into an editor to add a comment for the changes you've made. You can include the -m option and a string to do this at the command line.

Now you need to push your changes into your forked repository on github with git push -u upstream MyNewBranch

From the github website, you will need to create a push request using your branch.