Depending on the how real you want this flow to be, this might be quite complicated, 3D cameras, position keyframes, simulated gravity, particle emmitters, displacement maps all might be involved in creating a money waterfall. That's actually quite a bit to do for an experienced animator and you say you are fairly new to the program.
I'd strongly suggest you start here,
Getting started with After Effects (CS4, CS5, CS5.5, & CS6). Starting to learn AE with a finished project in mind is often a recipe for failure.
Johnny Cuevas, Editor
Thinkck.com
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.