Marco Alka has covered almost all the necessary commands.
Here's the one I use the most:
git diff --- shows difference in the uncommitted code changes happened from the last commit
git diff <file-name> --- shows difference in the uncommitted code changes happened on that file from the last commit
If you have lot of uncommited changes then above command will be hard to visualise. In those cases, I use gitk which is a simple UI based available on all platforms (apt-get/brew/windows)