A CAD program will do nothing that a sketch pad, scale, and a straight edge will do.... Like others said, the first thing you have to do is some planning, what engine and trans will you run, what are there dimensions? Need to determine a wheelbase so that you can design in an instant center that is acceptable.

I would say start with the drawing and parts planning first. There are so many variables that go into chassis design you are going to have to have a lot of your major components picked out and their dimensions before you can start on the frame rails....as well as a sturdy, flat, level, and squared surface to do the actual building on.....