I use Rhino. here is one of the first tracks i did while at the steep side of the learning curve.
Its pretty handy. you can design jumps and stuff and make shapes that a cnc router will cut.
Not something you are going to pick up in a couple of evenings.
You will be able to get the hang of, as Blue_Pinky mentions, DoubleCAD etc very quickly though. its a bit like an advanced 'windows paint' and very useful.
I have built many things with the aid of both 2 and 3d cad.
Have also used Zanthrax (pic below) as Wylie mentions. also useful. especially for indoor onroad.
