Seneca College in Toronto has a program geared towards exactly this — beginners to Mozilla development.
Check out some of these links:
Real World Mozilla
Fall 2008 Weekly Schedule
Some labs if you want to dive right in:
Lab: Learning Collaborative Development (Fall 2008)
Lab: Building Open Source Projects (Fall 2008)
Lab: Real World Mozilla – Source Code Reading
Lab: Working with Patches
Lab: Dive into Mozilla – Debugging Mozilla
Lab: Thunderbird Bug Fix
Lab: Modifying the Browser
Lab: Modifying Firefox using an extension
Lab: Real World Mozilla – First XPCOM Component
Lab: Real World Mozilla – Adding chrome to first XPCOM component
If you need any help, use IRC: #seneca on irc.mozilla.org. Don’t worry if you’re not a (Seneca) student, lots of people around the world (people from Seneca, people from Mozilla, and others) are in this channel, the purpose is to educate developers about open source in general and the Mozilla project specifically.
Good luck!