Getting started with jailbroken iOS development?
The Cydia FAQ is probably the best source of information on this topic. See http://cydia.saurik.com/faq/developing.html. Here’s an excerpt:


  Q: Where do I get started with developing for jailbroken iOS? 
  
  A: It helps to know something about developing for normal iOS. You might also check out the iPhone Dev Wiki.


The wiki it points to has several “Quick Start” articles: http://iphonedevwiki.net/index.php/Main_Page#Quick_start