ok- go into photoshop, or paint, and paint a black and white terrain map.
Something like- this.
It's not good, but I just drew it in a minute for the purpose of this short tutorial.
Then- in Maya, create either a Nurbs or Polygon plane. The more faces/patches you have, the more precision you get. You don't want to overload your computer, though. I'll just use 10 patches U and 10 patches V on a Nurbs Plane.
Now, assign it a new material. Open up the Hypershade, and create a new texture (create->2d texture->file). Open up your displacement map with the file texture node.
Now, middle-click and drag the material down to the work zone. THen, middle-click the file node and drag it onto the material. Then, when the mini-menu pops up, select Displacement Map.
Finally, grab the plane, assign the material to the plane, and keep it selected. Then go to modify->convert->displacement to polygons.
This should be your result: note the high poly count- more patches U and V would make the map be more precise and take up even more polys (up to millions). scale your plane if necessary.