Before you can start to play, you have to sit properly at the piano. Try to sit up straight and make sure your arms and hands aren't too high or too low to reach the keys.
You might think it's silly, but the correct posture makes a difference. If you sit too high or low you can put strain on your arms and hands. Some people find it affects their back, as well. So before you start to play, make sure you feel comfortable.

