If you're looking to just access the hard drive bay, it would be the metal cage on the bottom right. The cage is being held by only 1 screw.

The motherboard is being covered by a large vented metal shroud, you'll see it takes up about 1/3 of the space.

Remove the shroud by removing the surrounding screws and it should just lift right off.

You'll see that on the top left is the optical drive, and in the middle is the fan & heatsink for the CPU.