Next you need to come up with a viable idea for an app. And that begs the question, how do you know if your idea is viable?
That's a tough question to answer. There are currently 350,000 apps on the app store. Now, as I said in an earlier post on iPhone development, Apple is pretty loose about what sorts of apps they allow on the app store. There are a few exceptions though, and the two most notable are: no pornography and no apps that are carbon copies of others.
So what does this mean for you? It means you need to get out there and do some research.
There are already a zillion Twitter clients on the app store and unless your Twitter client does something totally new and amazing like posting tweets onto the back of a user's eyelids, then you better think of something else. Apple will turn you down.
A great place to start is a website called Mobile Orchard. They run a podcast and a blog about new iPhone apps. But here's their hook: instead of just clipping off opinionated reviews of whatever app they happen to come across, they search out true innovations in app design and then present interviews with the developers. Here's a great example: check out the second interview with Pete Schwab who created an app that tells you the temperature based on the chirping of nearby crickets! Seems esoteric, but for anyone wanting to work with audio on the iPhone, there's some great stuff there. Here's another typical post: an analysis of pricing on the app store. Super useful for anyone about to sell their app.
iTunes itself also has a ton of other podcasts about iPhone apps. Some of these are just folks who like to hear themselves talk, but some of them give useful reviews of popular apps and often include some technical discussion. Definitely useful.
You should also look to the popular media and see what sorts of apps people are buying. Time did a piece not so long ago on just this topic.
What this all boils down to is the fact that before you settle on an idea, you need to know what's out there already and what sorts of apps people are using. It is hard to please everyone, but don't forget that there are genre apps as well. While a beat box may not be as popular among the masses as Yelp, it might be to a fairly large group of music folks or djs.
So get out there, thin, research, re-think and create.