How does one start mocking up a mobile app? [closed]

Question

I have this idea for the "best app in the world". I know what it's going to do, the need it's going to fill, a high to mid-level idea of the code, and who i'm going to target. Now i'm ready to begin: I grab my pen, paper, thinking-cap, can-do-attitude, set the clock to GO-TIME--This app is gonna be awesome!......2 hours later, all i have on my paper is a big rectangle...

One would think that the mockup would be the easy part, but drawing a "simple & sleek" design harder than it sounds. Are there any techniques/advice that you can give on mocking up an app? Any references or rules-of-thumb guidelines you can provide would also be greatly appreciated.

Thanks in advance.