Posture is one of the first things you'll learn in piano lessons. It may seem unrelated and boring, but having the right posture will enable you to play to the best of your abilities. It also allows for greater comfort and increased stamina, which will help you out down the road. Here's a simple checklist for making sure you have correct posture:
1. Body is positioned on the front half of the bench or stool
2. Feet are planted firmly on the floor
3. Knees are slightly under the keys
4. Angle of legs is just over 90 degrees at the knees.