Zotero is free and open source, and we’re committed to fostering a community of developers around it. Toward that end, here are a couple places to start out:

• You can browse (and contribute to) the Developer Documentation wiki. We’ll be fleshing this out in the months to come, and we’d appreciate any help you’d like to offer.
• If you’d like to help with Zotero development or have questions that aren’t answered on the wiki, you can browse the messages on the zotero-dev mailing list or subscribe to join the conversation.
• Getting started, a short HOWTO for if you want to develop Zotero or Zotero plugins.