To cut safely, using your knives, on your work surface.
If you intend to buy:
Get a large one, no less than 30X40 cm. or 12X15 inches, in food-quality plastic. Contrary to a commonly-held belief, wooden chopping boards are not very good in the kitchen: it's quite impossible to get them completely clean, so germs stay on them, and when using with a sharp knife you end up making sawdust. See also this post about chopping board.