Develop an iOS App for Cydia
How can I start developing simple iOS tweaks for Cydia?
What's the difference in compiling an app for the official App Store and Cydia?
Answer
Developing a tweak is quite different from developing a self contained app. If you want to do it all yourself, read up on Objective C runtime programming. However, this is not how the vast majority of tweak developers do it, as there are several easier options.
If you have experience with Objective C (enough to be able to develop an app), there isn't a whole lot more you need to know to make a tweak. Dustin Howett, a developer who has made several tweaks for Cydia, made a tool called theos which vastly simplifies the process. You can read up on it here. There is also a very active IRC channel at irc.saurik.com #theos. Finally, you may want to check out the github pages of some popular tweak developers that use theos, so you can better understand the syntax (chpwn, DHowett, rpetrich).
Those three methods are how I learned to write tweaks, and once you understand theos it is really quite simple. theos also has systems for creating self contained apps as well as other plugins, so download it and see what is best for what you want. A quick google search will also turn up several useful theos tutorials (1, 2). Good luck!
One final thing to note is that you may want to hold off on developing for a couple weeks, the reason being that Apple may be releasing iOS 5 soon and things could change, as well as the fact that everyone on the #theos IRC will refuse to help with problems pertaining to iOS 5.