To scratch off cards to win a prize, first you must read the directions for that specific card. The directions should point to where you should "scratch." Use a coin to rub off the scratch-off coating, and if you uncover what the directions indicate is a wining ticket, you win.
Many of these games will have you scratch-off various items to uncover matching items. Some will require you to scratch the entire card, while others will restrict you to scratching off only a portion. This is why reading the directions is so important. You could have a winning card that becomes invalid if you do not read the directions correctly.