Before we can start building our custom field we need a vanilla Drupal site to work with and a skeleton module. This lesson will ensure you've got Drupal 7 up and running and walk through creation of a basic .info file and .module file for the module we'll be building. If you're already familiar with Drupal module development this lesson can likely be skipped and you can simply download the attached starter files, add them to an existing Drupal site, and continue on with the next lesson.
Grab a fresh copy of Drupal 7, and install it. If you need a refresher on installing Drupal checkout this series.
You'll also want to download and install the devel module as we'll make use of some of the debugging functions in provides (namely
dsm()) in later lessons in this series.
Alternatly, you can grab the .zip file under the companion files listed on this page which contains Drupal 7, and a database dump you can import to get started.