Help with GUI design

Helo

My friend ask me for help with GUI design. We want me to do GUI for some app he is doing, so basicly he needs first a realistic on/off button and color picker. Both needs to look as much realistic as they can and must be done in Photoshop.

Now i had never done any GUI for android/apple, is there anything i need to be careful? Any literaure (short) that i need? Wher to find good GUI examples for Apple/Android so i can find some inspiration or examples?

Also would like to know what resolutions are in use for GUI in apple, android devices? or some templates of iphone, and android devices so i can create a sample and show it to him