Creating iPhone® web apps is a technical task which requires advanced computer skills. This explanation will assume you have these skills and will instead give an overview of the process of developing an iPhone® web app and getting it into the app store.
The first rule of successful iPhone® web apps is to create something which solves a problem. You should create an app because there is a need for it, not simply because you've found a way to do it. As a guideline, look for a subject area where there are already several apps available, showing there is a good general market for the subject, but where no existing app meets the specific needs of the one you are planning.
To develop iPhone® web apps, you'll need to join the official Apple developer program. This costs $99 US Dollars (USD) in most cases. There is a $299 USD Enterprise program, but this is only needed if you want to distribute apps across a corporate intranet. Commercial apps aimed at the public can be covered by the $99 USD program.
It's also wise to download the iPhone® software developer toolkit. This includes a variety of tools for making sure your app is fully compatible with the iPhone® and iPod touch®. It includes an emulator so that you can see exactly how your app will look and work on a real device.
The process of creating the app involves three main stages: design, programming and testing. All are vital and it is worth planning as much of your work in advance to avoid unnecessary hiccups. At all stages, you should pay close attention to your intended market and the needs of customers. Independent testing by users from your target audience is particularly important as this will pick up problems you will not spot yourself.
Once created, iPhone® web apps must be submitted to Apple for approval. This is a two-part process. The app will be tested to make sure it will work on the iPhone® and iPod touch® and will not pose any security risks. The app will also be vetted to make sure it does not contain unsuitable content. There is some leeway for apps which will be marked as adult only. Some content will always be rejected, while other approval decisions by Apple may appear inconsistent, so it is worth checking in advance if you expect to encounter any problems.