You'll see four windows open on the screen. Hover the mouse in the top-left corner of a box to see its name. The one you want is the top right window, which is named Top (x/y) window. This window is an overhead view of your level. Head over to the toolbar on the left, click the white cube, and then drag out a square in the (x/y) window. You actually want a flat surface and not a cube. In one of the bottom two windows, which show the sides of your level, drag the top of the object down until it's only 64 units tall. Click Enter and you have a floor.
If you want to check out your work click the red camera icon or press z. WASD and the mouse wheel will fly you around. We're going to add walls. In the camera window, click on the floor and it will turn red and yellow in the other window. In one of these windows, hold down Shift and then click and drag to create a copy of it. You can now make it thinner but taller until it's a vertical wall, shift-click again to copy it and make a facing wall and then repeat until you've got a wall on every side and a ceiling on top.
Assuming players are going to be shooting at each other while they're playing with your map. To help them when they die, we're going to build a respawn room. Using what you've just learned, create another smaller room inside the first room. Enter 90 in the z box to flip a wall around 90 degrees. Make sure you leave an exit from this room, or it'll be a really boring game. Now, in camera view, hold down CTRL and click on each wall of the spawn room to select all of them at once. Once this is done, just do the Shift-drag clone trick I showed you earlier to create a second spawn room and then place it at the other end of the map.
