Once you walk up the wide stairs from where you start, you'll see a huge, spiraling staircase of stone. In front of it is a lever. Pulling it will rotate the staircase. But since you'll be doing that anyway, wait until you get back to this room later to pull the lever - you'll have to later anyway, so to avoid wasting time, don't pull the lever until you're back in the room after seeing and catching the first of two gargantuan balls of green light.
This is also a good time to just leap off the top of the spiral staircase and get the Jumping Around achievement. You'll have to get down somehow anyway to pull that lever, so why not challenge gravity while you're at it? Don't let that height scare you...