First drag your mouse from somewhere in the upper, left section of the screen to the lower, right section of the screen, a red square should appear. Now, move the square around by clicking somewhere in the square and moving the mouse, then resize by clicking and holding the mouse down a bit outside the square on the side you want to move and dragging it until it has reached the correct size. Try to make it fit the 128 marks on all sides. Also try to look at the square in 3D by right clicking somewhere in the grey 3D window and scrolling back with the mouse, maybe you have to move the mouse around a bit as well, try to get used to navigating in the 3D view because being able to do so will help you a lot in the future. When looking in the 3D view you will see a cube, which is rather thick, this cube is going to be the floor of our room, but, its still a bit thick, right, so lets make it thinner.
As you can see in the 3D view, the floor still looks weird, this is because the floor isn't textured yet, so, start by selecting the floor again by holding down SHIFT and clicking on the floor. Before applying a texture to the floor, you should caulk it, this means that you apply a caulk shader to all surfaces (don't try to understand that yet), which is easy to do, so once you have selected the floor, press the caulk button in the toolbar and you will see that all surfaces in the 3D view turn pink with the words caulk on it. No worries, it won't look like that ingame, in fact, everything which is caulked is invisible ingame, so what you just did was making the whole block invisible.
Now to the real texturing which means that you apply an actual image to the upper surface of your floor. First of all deselect everything (ESC) . So, start by double clicking a texture folder in the texture folders list, then wait a moment and textures should appear in the grey area nearby. Click one of those textures, then hold down CTRL+SHIFT and click your middle mouse button (scroll wheel) on the surface where you want to apply the texture, this is best done in the 3D view. When done, one of the surfaces should have a texture on it, and all other should be pink... if not, just repeat this and it'll work sooner or later.
If you want, you can train to navigate around in the 3D view a bit more, and see if you can change the viewpoints, try to texture the floor with some different textures, select and deselect stuff, and so on until you know for sure what you are doing. This is important because the next chapter will build on stuff from this one, i.e. I won't explain step by step how to texture stuff, or how to resize or move brushes.