If you've just installed Windows 8, the easiest way is to re-install it. It's helpful to have the disk already set up with a GUID Partition Table (GPT), but a blank disk will work, too, and I gather it's possible to have Windows wipe an MBR disk and set it up with GPT (although this will effectively erase any existing data; it's not a lossless conversion). The trick is likely to be to get the Windows installer to boot in EFI mode. Most EFIs offer a boot manager that lets you specify the boot mode of a CD, DVD, or USB flash drive, so you should be able to use that to force the correct boot mode. If the disk is prepared with GPT and at least one partition, the Windows installer will then complain if it's booted in BIOS mode, which is better than getting through the whole installation only to discover it's a BIOS-mode installation.
If you have data you want to preserve on the disk, you can do an in-place conversion. The process is described here; however, that page describes the process using DUET, which is a software EFI implementation designed to be run atop a standard BIOS. Using DUET complicates the procedure, so you'll be able to ignore much of what's on the page I just referenced.