I can make some development philosophy suggestions, based on my experience. For the technical issues of getting started, you can search around here or in Apple's excellent documentation.
Make your first application free. I decided to do that, and it's worked out very well. A free project is a great way to become acclimated to the frameworks, tools, submission process, etc. Good free applications can also get you a lot of exposure and build goodwill. For example, my application has been downloaded over 340,000 times worldwide. Hopefully, I've made a favorable impression on many of those users, who may be more likely to purchase something I write in the future. Even better, open source your program and allow others to learn from your work. You'll get a tremendous number of hits to your blog or site just from other developers looking for code examples.
You asked for how to have fun, well I can tell you that I've greatly enjoyed hearing from educators and scientists around the globe. By releasing the code for the application, I've had a chance to talk to a number of great Mac / iPhone developers and learn from them. I've had more fun doing Cocoa development on the Mac and iPhone than on any other platform I've worked on.
The next thing I'd encourage is for you to challenge yourself in the development of your application. If you can write it in under two weeks, don't bother. Almost all of the low-hanging fruit has been plucked in the App Store, with decent free versions of most ideas. A talented developer can differentiate himself from the crowd by writing an application that's difficult to do a good free implementation of. In particular, I'd love to see more applications aimed at scientists, engineers, and even students.
If you're worried about the limited exposure that a higher-priced, quality application gets now, I don't think that will be as much of an issue in the future. Significant improvement have been made to the App Store since launch, and I believe even bigger ones are to come that will make it much easier for users to pick out the quality applications from the garbage. Hopefully, the market will sort out in the way that it has on the Mac, where the polished, easy-to-use applications become the most popular.