I studied the wine developer's guide and went a bit through the code. I would like a suggestion, a bug to start understanding better the structure of wine and familiarize myself with it in order to be able to contribute more in the future.
Often, picking an application that you yourself run is the best way to work through the code, as it'll keep you motivated to understand.
Another suggestion is to look through bugzilla for bugs that have the patch keyword. Trying to understand what the patch does, and why it hasn't yet been applied, may give you the insight you're looking for.
Good luck, --Juan