Either way. You can install Windows using Bootcamp Assistant or you can install Windows using a virtual machine like Parallels, Fusion, or VirtualBox.

You need to buy Windows.

If you are using a virtual machine you also need to buy Parallels or Fusion or use the free VirtualBox.

If using Bootcamp Assistant it is already on your computer in the Utilities folder.

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It depends on your needs. For example, if you need Windows to work with applications that require a high performance (play games. video and photo editing, etc.), use Boot Camp.

If you need Windows for light tasks, you just need a virtual machine with Parallels, VMware Fusion or VirtualBox. You can read all the advantages and disadvantages of both of them in this website > https://discussions.apple.com/docs/DOC-3321

If you decide to use Boot Camp, follow Apple's steps > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.8.pdf

A somewhat common approach is to do both.

You first install Windows natively (Boot Camp) and then after that is done (but probably before activated) then connect Parallels or Fusion to use the same Windows partition for its use.

That way you can dual boot and run it natively or run under a VM inside / side by side with Mac OS.

That is the one feature that I don't believe you can do with VirtualBox.

You can find deals on Parallels and Windows probably.

The next choice, Windows. I use 8.1 which will be out probably in fall, a couple months and prefer 8.1 over any of the older and previous editions.

You want 64-bit even if and especially if you have 4GB of RAM it handles memory better, and of course if you have more.

Boot Camp: Frequently asked questions about installing Windows 8

http://support.apple.com/kb/HT5628

Products Affected

Boot Camp, Windows 8

General installation questions

http://support.apple.com/kb/HT5639

Which Macs support Windows 8?

• MacBook Air (Mid 2011 or newer)
• MacBook Pro (Mid 2010 or newer)
• Mac Pro (Early 2009 or newer)
• Mac Mini (Mid 2011 or newer)
• iMac (27-inch, Mid 2010 or Mid 2011 or newer)

For more information, see this article.

What are the System Requirements for Windows 8?
Please see this article.

How can I install Windows 8 on an eligible computer?
Use the Boot Camp Assistant. The assistant will partition your internal hard drives and install Windows 8. For more information on Windows 8 installation, see the Boot Camp Installation & Setup Guide.

https://support.apple.com/kb/HT1461

Helpful Apple Support Resources (Forum Overview)

• Boot Camp Support 
• Boot Camp Manuals