There's not much to say here. Hop on over to the VirtualBox site, download the installer and run it. During the install you'll not only get the UAC popup asking for administrator rights, but also several popups asking to install device drivers. This is normal, as it must install virtual networking interfaces and such.
Create the Virtual Machine
When VirtualBox is finished installing, you should see a window that looks like the screenshot above. Click on the New button, give the VM a name and select Linux and the type of Linux distro you have. Be sure to give it a fair amount of memory, but not so much that your host OS runs out of memory. I usually stick to 2 gigabytes, it's enough for most tasks and on my desktop computer it leaves lots of room for Windows and other Windows programs.
The rest of the options just leave as default. I like to give it a bit bigger hard drive (20 gigs), but that's up to you. The default 8 gigs is probably OK, depending on what you're using Linux for.