The Codex has a page on how to contribute to WordPress. Some other ways to help:
- Help others with questions. On the WP.org support forums, here, or on other sites, it does not matter. Pick a question that has been open for a while and try to solve it. I have a tool to quickly start experimenting with code, and most of what I know about WordPress comes from answering questions here. It even made me write my first plugin for other developers.
- Improve the Codex. The same reason as answering questions: to explain something you must first understand it yourself. And the Codex is in need of some serious rewriting.
- Look at code written by others. You can volunteer to review themes: this will teach you about the coding standards and you’ll learn interesting ways to use WordPress.
- Write code yourself. If you are not confident enough to write complete patches, you can also test new features or confirm new bug reports. By trying to isolate the bug you will also learn how the inner code works, and gain respect among your peers.