This one is pretty short and simple, but still worth a mention mostly because I see server administrators struggle all the time to find things inside their Start menu. There are two quick places from which you can open the GPMC. Immediately, when clicking on Start, you will likely see some live tiles on the right side of the Start menu, and one of those is the Windows Administrative Tools. Clicking on that tile will open a folder with links to all of the administrative tools, including Group Policy Management.

Alternatively, you can open the folder for Windows Administrative Tools that is on the left side of the Start menu, which will expand all of the tools right there, inside the Start menu. From there once again, it's simply a matter of clicking on Group Policy Management:

As easy as this option seems, in my experience, the new Start menu can be a bit of a resource hog and therefore slow to respond on many systems. So, while this seems like the logical place from which to launch applications, I find it to be the least likely candidate for most IT personnel.