It's very simple. Have a copy of Windows XP, Vista, 7. Start up Bootcamp. It will partition your drive at your desired allocation, then it will tell you to insert your Windows disk and install windows. After this it will install Mac/Window drivers so Windows can interact with your hardware. In Windows you'll find an Apple control panel which will update your drivers from time to time.
After launch Bootcamp by holding down Option on Start and pick the Windows partition to boot from.