Deus Ex Level Infos are very important, and you must have one in every map. Select PlayerStart and add it to your map. You'll be launched into your level. So maybe you know enough to create amazing levels already. But I'm sure most of you don't.
Lets add a basic light fixture.
You'll see that the builder brush will have shrunk. Also it will be in the centre of the room. Use either 2D view on the right, select the brush by clicking on it's edges. Then hold Ctrl and drag the brush to the ceiling. Now click Add .