You've got a couple of options when it comes to running Windows on your Mac.
The first option, as you mentioned, is running Parallels for Mac. This will allow you to run Windows from inside the Mac OS X system, which is ideal if you need to access Windows programs and Mac programs at the same time.
The second option is running some software that comes included on the Mac called Boot Camp. This program allows your Mac computer to load Windows instead of loading Mac OS. While Boot Camp is free, you'll need to restart your computer any time you need to switch between the Windows system and the Mac OS system.
Regardless of which option you choose above, you'll also need to buy a copy of Windows 7 to install onto the Mac computer. It'll need to be a full copy, as you won't be able to activate an Upgrade copy on a computer that doesn't already have Windows installed. I also recommend Home Premium over Professional and Ultimate; the latter two offer extra features which often aren't necessary for most users. You can read more about the differences in Windows 7 versions here and decide if you'll need any of the additional benefits the more expensive packages offer.
Hope that helps!
