What is the easiest way to make good moulds for Vacuum forming?

I was thinking of making them out of wood or aluminum.
Can any one give me some tips plz?

You usually can't go wrong with wood, just be cautious of the materials you use. If your "buck" (positive mold pattern) isn't clean and has the grain it could get picked up in the vacformed piece. Metal also works however can be a bit on the expensive side if there is a lot of detail. You may also want to try using resin or fiberglass, again just ensure that your "buck" is sealed and clean, as all the little imperfections in the surface will get picked up in the plastic you form.