One should have a clear idea regarding what sort of application he/she wants to build. Do remember that your idea should be unique and bring something attractive to users otherwise it will be a waste of your time resources because if you are not going to introduce something new to the audience they will be download or purchase your app.
To be sure that the app you are trying to make is not already in use by the audience, you should search the web well. Be certain that your app has a rich interface and is going to make a considerable contribution in the iPhone app world.
Before starting your work formally propose the idea of your application to ensure its acceptability by the App Store.
You should design a framework about how you will build that application of yours. You might face several problems while getting approval of the App Store but remember hard work is the key to success.
Many developers forget to code their app, which is very important. You should know how to code your app, if you do not, outsource the services of an exiting iPhone app maker to complete the task.
You have to pay USD99 to get registered to the Apple Developer Network. To get this done, you should go to iOS Developer Program and create your account by registering yourself there.