If you've thought about contributing or reviewing code, but aren't sure where to start, there's a good blog post from a GNOME OPW intern that I'd like to recommend. It was written for OpenStack, but the advice is applicable to MediaWiki contributors too, since the two foundations have common tooling and practices related to code management and review.
Reviewing an OpenStack Patch http://anteaya.info/blog/2013/03/21/reviewing-an-openstack-patch/
There is, of course, excellent content on MediaWiki.org on this topic, but this is a nice reinforcement.