developing to ipad. what are the requirements?

Hi All,

Quick simple question. I am looking to develop some applications that i want to package and put on ipad/iphone.
What do i need to do this. Do i need some sort of developer license from apple? Are there any costs?
A simple list of requirements would be useful, just so i know the steps of getting a packaged project onto an ipad before i begin down this route.

Many thanks,