Having a good posture allows you to practice comfortably and leads to better piano playing.
Maintain a neutral spine and relax your shoulders. Your legs should be uncrossed and flat on the floor or on a footstool. Check that your elbows are positioned around keyboard level, and that you have a 90 degree bend between your forearm and upper arm. Relax your muscles; your body should not be tense.
Check periodically to make sure you keep proper posture.
A good way to find the correct hand position is to pretend you are holding a ball in your hand. When you flip your hand over, keep your fingers in this curved shape.
Piano should be played on the fingertips, except for the thumb, which is played slightly on its side. Your wrists should be relaxed and slightly lifted.
Check the height of your bench. When you reach your hands out to the piano, your forearms should be parallel to the floor. Adjust your seat, or sit on a book or two if necessary.
Distance is also important. Your bench is too close to the piano, if your arms are being pushed in towards your body. It is too far away, if you need to stretch your arms out fully to reach the piano. The correct distance allows you to reach the keys comfortably.