Sit up straight and relaxed. If your seat can move up and down, adjust it to a comfortable height. If your seat is too low, sit on a cushion.
You may need to rest your feet on a stool if they do not touch the ground. If your feet do reach the ground, keep them flat on the floor.
Always curve your fingers as if you were holding a ball. This helps keep your fingers at the same level, as shown in the drawings here.
When you play the keys on the piano, use the tips of your fingers, and the side of your thumb.