Please note I just threw this together in like 30 min so it does not look like much. I will leave it up to you to design your scene. But it does what you want... Okay... here we go.

Warning!!!

This will most likely consist of several posts:

First create a Poly Plane (If you don't know how I think this project is a little too advanced for ya )

Now in the attributes editor set its width to 5 and length to 5.

Next set the subdivisions to 50 for both width and lenght.

With me so far?

This will create a plane which you can use for your desert surface. To make it a little more like a desert you can apply a bump map or a displacement map. But if you need a quicker way use the sculpt polygon tool to make your dunes.