Google Code-in/MusicBrainz editing
MusicBrainz is a pretty complicated database, with a steep learning curve, but there are some documents to help you out!
Check out the Introduction to Editing, as well as the different Style guidelines. Don't worry about getting lost, each page has an overview of the other pages at the bottom
We also have a sandbox test server at test.musicbrainz.org too, there you can create or log in as anyone, (with the password "mb") and test and break as much stuff as you want. No live data is altered on the test server, so you don't have to worry about breaking anything there. Feel free to wreck havoc!
When doing tasks, make sure you're using the main musicbrainz.org server though. for beta releases we have beta.musicbrainz.org but you don't have to worry bout that.