Bitbucket and Github both provide a quite intuitive interface for reviewing code. You can simply look at a list of recent commits, and find a particular line to add a comment.
If you are new to git, I'd recommend you find an existing project and try this out.
- Find the commits tab - https://github.com/jaz303/tipsy/commits/master for example
- You can see a number of commits, with references like 3545aa6ae2 - these are hashes that uniquely identify the change.
It shows the commit message, author, as well as date/time.
- Viewing one brings up the review user interface.