One method would be to take your final animation composition and drag it on to the new comp button. Create a solid and turn off the eyeball for it. Now use the pen tool(g) to cut a mask that you want cut out and change the mask mode to subtract(turn on the eyeball to see what you created). Last change the track matte of your animation layer to alpha mask. Then render your composition to a format that supports an alpha channel. Like Quicktime_Animation. Under the lossless dialogue you will need to change the color from RGB to RGB+Alpha. Here's a quick example project for you: 4827_pictureframemattefolder.zip
Given that you are a new to AE, I would suggest you look at this PAGE. It's a collection of resources collected by Todd Kopriva. Try the getting started video tutorials he posted links to in point 4, you will find AE a lot easier once you understand the basic tools.
Johnny Cuevas, Editor
"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.