@ simm0t is correct, you need a paid iOS developer account. You can get one of those on developer.apple.com .
In addition, if you simply cannot go through this process for any reason, you can develop your application with a dummy certificate / preparation (you can find them on the Internet) and download it to the device with a hack, and then install the application using Installous. While I was a student and couldn't afford to pay the IOS dev account, I was able to learn how to make iPhone apps and test them on my iPod touch.
source share