It's true that the existing codebase is too huge. But you do not need to figure out the entire code base all at once.

Pick some easy bugs from the bugtracker (some bugs are flagged a good first bugs, those are nice starting places). These bugs will help you set up and get used to some of the code. Once you do that, move onto gradually tougher challenges.

Whenever you get stuck, feel free to ask about that in the IRC. Believe are eager and willing to help any new developer, so you won't have much of a problem. Get in touch with your mentor. He/She will be the best person to help you understand the code base.

Once you solve a few bugs and are comfortable with working with the code, you will find out that the open source community is a beautiful place.