It depends on the thickness you want, but I would suggest nothing less than a 50mm layer of concrete on top of a 50mm layer of hardcore/sand. For this thickness, you will need roughly 0.35 Cu M. of concrete, which is about 20 x 80lb bags.
The hardcore can be bought or you can use any crushed rock or rubble you have to hand (make sure it doesn't have any metal or plaster in it).
There are some good guides to using concrete
here
