Although the classic posture is to sit cross-legged, what is more important is finding a position that is comfortable for you.
Instead of sitting on a cushion or on the floor, you can also sit in a chair. Your feet should touch the ground. If they don't place a stool under your feet.
Rest your hands palm-down on your thighs or knees.
Energy is thought to flow better through the body when you are sitting upright, so it is important to sit up straight. It may help to imagine your body being pulled up from the top of your head so you are upright without being stiff.