git clone $YOUR_REPO_URL after the above succeeds, you'll have a new directory that you have to enter; in this directory, you can complete the assignment whenever you finished coding, add your changes to the index; to see which files are changed, use
git status; to add them to the index use
git add [...] make a snapshot of your current status quo using
git commit -m '$YOUR_COMMIT_MESSAGE' upload your new change snapshots (commits) to the classroom platform
For further information on using distributed version control with git, see the overview page on DVCs or the git cheat sheet.