iOS Concept & Design

First: Get an iPad yourself and use it. Use it privatly at home. Again: Use it! (sic!)

Have a look at the native app designs: Mail, Safari, Contacts, Calendar, ...
Apple iPad built-in-apps
Understand their approach and pay attention to the transition between portrait and landscape

Read and understand the Apple iOS Human Interface Guidelines

Keep in mind that you are designing for a multi-touch device
Touch Gesture Reference Guide

Now what is the starting point?
Design an application from scratch?
Display data from an existing data source?

The definition of use-cases is a good starting point.
If you have existing data it is very useful to know all data available and the connections between the data. Ask for an entity relationship model.

Create a functional design, e.g. on paper
Free printable iPad design template stencils (PDF)
iPad Stencil Kit (Pen & Paper)
iMockup iPad App
iPad Sketch Elements AI
Omnigraffle iPad stencil

• collaborating on ideas
• wireframing
• storyboarding
• interaction design
• presenting ideas to clients

Check your functional design with the developer, too!

Some more collections:
GUI Stencils and Wireframing Kits for iPad App Designers
iPhone and iPad Development GUI Kits, Stencils and Icons

Export graphics required as PNG. Agree with the developer on file naming conventions.