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.