You can contribute in one of 3 ways:

1) Find an open issue in a project:

Checkout the list of open issues for something you can contribute to. If you find something you want to help with, change the status of the issue to "Started". If you have questions, email the project lead and/or person who filed the issue. 

2) Contribute your own idea to an existing project:

Have an idea for an improvement or new feature to a project? Email the project lead to run it by them. If you get the go ahead, then create an issue to track it, and get started coding!

3) Want to contribute your own project?

Have your own tool that you want to open-source with edCode? Checkout the list of requirements for edCode.org projects, then email dave@edcode.org to discuss it. 

The code is hosted on code.google.com, in a Subversion code repository. If you plan to contribute, you'll first need to get an account setup in the repository. Email dave@edcode.org for this, and include which email address you'd like to use. 

Once you have access, see this article on how to checkout all the code. You'll need to download an svn client for this.  

Also, see any specific instructions for the project you're contributing to (also here in the KB). 

Make sure you've updated to the latest code ("svn update"), and then get started developing! 

Be sure to see any specific instructions for your project on how to test your changes. 

Once you've completed your changes, run "svn update" once again to get updates on any changes other may have made to the same project.  You may have to resolve any conflicts, if those changes were made to the same sections of the code you modified. Test your changes again.

See this KB article for information on how to commit your changes.