Something that commonly affects musicians is arm/hand strain when playing. This can affect beginners and advanced pianists alike and is something that I have suffered from in the past. I also tend to see it in new pupils as they are not used to the positions adopted when starting to play. Problems such as Repetitive Strain Injury (RSI) and tendonitis can arise if bad habits are adopted long term. Thankfully, there are ways to avoid this with some simple adjustments to posture and seating position when practising.
The way you sit is incredibly important as being too high or low can hurt your back, shoulders and arms, especially if playing for long periods of time. Preferably, the arms should be horizontal with the elbows just below the level of the keyboard and the hands should be relaxed. I used to carry a lot of tension in my shoulders and upper back and so I now consciously try to keep my back straight and my shoulders open and dropped down instead of hunched. A lot of people tend to think that good finger dexterity comes from tensing the muscles in order to strengthen them but you will find that the hands become stronger and more flexible if they are kept relaxed and heavy. Lastly, the most important thing you can do to avoid painful arms when playing is to ensure you have regular breaks when practising and try to play for short periods of time throughout the week so your piano lesson is not too much of a shock for your muscles! Keeping healthy also helps and I used to do Pilates and the Alexander Technique to strengthen muscles and help my posture.
Following these simple steps and getting in to good habits right from the start can avoid any problems later on and will keep your piano playing enjoyable and relaxing.