[support] Customizing Drupal Commerce

Hi, I am looking at customizing Drupal Commerce. I've been a LAMP dev for a while however I am very new to Drupal. I have a ton of questions. First let me say thank you for all your help and guidance. I am wondering the best way to proceed? Hack the existing commerce modules or create other modules that interact with the existing commerce modules? Thanks again for all your help!! Keith