Module developer's guide
Developer documentation can be found at http://api.drupal.org and in the remainder of the Drupal developer's guide below.
- api.drupal.org documents the Drupal APIs and presents an overview of Drupal's building blocks along with handy examples.
- The Drupal developer guide provides guidelines as how to upgrade your modules (API changes) along with development tips/tutorials.
- See also Maintaining a project on drupal.org
You should also become acquainted with Drupal coding and documentation standards as well as Writing secure code.