I have already installed Windows on my Mac, and I’m hoping that I can run that partition from VirtualBox. I realize this is probably a stupid question, but I’m new to VirtualBox. Thanks!
If you still looking for an assistance on that matter check out latest MakeUseOf guide.
DOWNLOAD Computer In Your Computer: How To Use VirtualBox
In general it is possible to run an existing Windows installation from an existing partition but there are a few steps needed and a lot of caution (as in you definitely want to do a backup of your data).
You will need to follow this guide to prepare Windows for VirtualBox
You will need to substitute points 3,4,5 with the following guide on creating VMDK
I have never tested this myself! If you have any questions or need help I suggest to join the official VirtualBox community forum since those people are more capable of helping you.
Another point to mention is that once your installation is converted to VirtualBox you will most likely not be able to boot it via Boot Camp.
Why don't you try this step-by-step tutorial, complete with screen shots?: