Learn how to design an app by reading as much as you can about user experience and graphic design, particularly as these relate to mobile devices. You need to think about the physical realities of the gadget and how it is used.
For example, the screen is much smaller than a desktop or laptop display. You cannot put as many images or blocks of text. Chop long paragraphs into several shorter ones. Navigation options are often hidden away to reduce screen clutter. A button can make it appear or disappear, usually placed at the top left corner.
Most people also read in portrait mode because this is comfortable with one-handed operation. Other control buttons should be placed at the bottom where they are easy to reach with the thumb. These should be finger-friendly with good size and spacing. Make them pop with a vivid color.