You need a copy of Windows 7 8 or the newly released 10. The copy must be new and have a License Key. Since your mac doesn't have a Disk Tray you have to create a disk image of the windows copy.
> Create a disk image of your Windows Disc
When you purchase Windows, it will come as a disk image file (ISO) or as an installation disc. If your copy of Windows came on a DVD, you should create a disk image of your installation disc.
> Open Boot Camp Assistant
Boot Camp Assistant walks you through installing Windows on your Mac. Open this app from the Utilities folder in the Applications folder and follow the onscreen instructions to automatically repartition your startup disk and download related software drivers for Windows. When prompted, connect your USB flash drive.
> Format your Windows partition
When you complete the assistant, your Mac restarts to the Windows installer. When you're asked where you want to install Windows, select the BOOTCAMP partition, then click Format.
Install Windows
> Follow the onscreen prompts to finish your Windows installation.
> Restart your computer
After Windows is installed, you can switch between OS X and Windows. Use the Startup Disk preference pane in OS X, or the Boot Camp system tray item in Windows to select your startup disk, then restart your computer
Cheers
Jay