Here's a quick example of making a change to the repo using the command line. Here I've made a change to the HW2 key. If this is your first commit, you may want to set your username and email. You should only need to do this once.
In terminal:
Before committing you can run git status, which will list under the heading “Changes to be committed:” all the files which will be affected. After committing you can run git log, which should then list your commit.
(Thanks to Thomas Hansen)