I checked out the page you linked me to, but I still don't understand. Plus, I'm running very short of time. I probably should've already said that I have about 10 minutes of video. I don't know if this is what's causing trouble.
Okay, so I went to Composition > Save Ram Preview, and was able to save half of the video. I still need the other half. Reduce resolution or add more ram? How do I do that?
Once again, you will NOT be able to ram preview all 10 minutes of video because that takes more ram than you have. AE is not a non linear editor and it's designed to create shots not edited movies. You can render movies that are longer than the ram preview but the ram preview length is limited to the available ram.
Check out the third video on this page about rendering.Basic Workflow