Joomla! provides an excellent opportunity for developers and other third parties to extend the capabilities of the CMS through a variety of extensions. Below are some resources for assisting you.
There are several tutorials and code samples available for your review. You can follow steps for writing a Joomla! component, module, or a plugin. These are just a few of the many valuable resources available in the Developer documentation.
In addition to the documents there are also various code samples available. You can view them in the main GitHub repository. A new extension development series for Joomla! 3.0 has also been addressed in the Joomla! Magazine and other associated websites.
After you have created your component, module, or plugin, you can list it for free in the Joomla! Extensions Directory (JED). This will help you gain exposure for your extension as well as provide a central location for your extension to gather reviews and other helpful tracking information. With over 7,000 extensions the JED provides a vast number of extensions to the community.