I would also add that there are several places to find ideas on what to work on:
We use a bug tracker to catalog all current issues, ideas, and feature requests for Octave. Many are substantial, but some are minor function implementations, Matlab compatibility fixes, etc.
The Octave Wiki does have a project ideas list. Some of these would involve quite a bit of experience with Octave, and I'm not sure if some ideas may be a bit outdated:
Finally, there are some good suggestions over on Octave's Google Summer of Code page, in particular some of the 'how to get started' suggestions:
(if it interests you GSOC applications are open through April 9. A small spring semester project before GSOC wouldn't be a bad way to get up to speed on Octave code development).