I am going to try to recreate my Thanksgiving Room for you.  This involves many layers, so remember to name each one, and save often!  Keep in mind, this is just one of many ways to create rooms - I've tried to keep it simple!

First, start with a transparent image, 600 x 600.   Decide what color your walls are going to be. Add a new layer, name it left wall.  I chose a pale green (red 223, green 237, blue 227).  If you are going to want a texture or a pattern for your walls, wait until you have the wall completely setup.  This will make filling in blank areas a bit easier.

Next, divide your room in half with the selection tool.  Double click on the selection tool, and a box will come up.  Enter these values: left 0, right 300, top 0, bottom 600.  Flood fill this selection with your choice of color.

With the left wall still selected, go to Image / Deformations / Perspective Horizontal. Make sure auto proof is checked.  Enter the value 80 (it helps to view at half size sometimes for proportions) and only look at the bottom to visualize the final result.  Don't worry about the top for now.  Ctrl Z if you aren't happy with it.  Leave the left wall selected.

Create a new layer and name it left white.  Flood fill this layer in the area of the selection for the left wall.  Move it beneath the left wall.  Keep the left wall still selected.

Create a new layer, name it back wall.  Invert your selection. Flood fill with your color choice.

With the back wall still selected, click on the mover tool and move the back wall up until it looks like it "fits". Move it over the left wall layer as shown in the image below.  How far left you move it will be up to you, but keep in mind that there is going to be a doorway on the right hand side.  Make sure your back wall is arranged above the left wall layer.  Take  note of how the two pieces of wall meet at the bottom of the center.

With the back wall layer still active, use the air brush to fill in the area to the right that is now blank, just the right, not the bottom.  If your brush goes over the bottom line, don't worry about it.  You can use the selection tool while the back layer is still active and delete that part.

Now activate your left wall layer.  Use the air brush to fill in the blank area on the top.  Adjust the opacity of the left wall.  This example is at 70% so you can see the difference and the definition it adds to the room.  This is also why I placed the white behind the left wall.  You'll notice this emphasizes the corner and makes it look more "real."  You may need to move the left wall after you set the Opacity, just air brush any areas after this that are not covered.  If you look at these two captures you can now see how the angle of the walls has been formed and then defined by changing the opacity on the left wall layer.

Now that your left and back walls are in place correctly, you can choose to either add a texture to the wall or use a pattern of your choice.

The next step is to add the floor.  I'm going to give you a couple of options for this part.