This article lists common questions for WordPress core developers, as well as developers of WordPress plugins and themes, and refers you to articles that contain the answers. There is also a list of resources at Developer Documentation, which might be of help.
Where do I sign up?
Can I help by translating WordPress?+-
What is Subversion?+-
How do I report a bug in a plugin?+-
How does the GPL affect my freedom in licensing my plugin or theme?+-
Most plugin and theme authors choose to license their plugins and themes under the GPL. Also, keep in mind that if your theme or plugin incorporates code from another theme or plugin that uses the GPL, you will probably need to license your theme or plugin with the GPL as well. If your theme or plugin is completely independent of other WordPress code, themes, and plugins, then you can probably license it however you would like.