Visit Stackoverflow.com, or the IRC channel(freenode.com).
The best way (the only way?) to contribute code is to submit a pull request. We have a few guidelines for you.
If you're working on a big ticket item, best to check in first on the #meteor IRC channel or email us at firstname.lastname@example.org. We'd hate to have to steer you in a different direction after you've already put in a lot of hard work.