Shoulder
Can your left and right arms reach the same distance above your head? Do your shoulders round forward most of the time? Is one shoulder hitched higher than the other when you look in the mirror?

These questions are a great starting point to help you recognise any limits of range or weaknesses in your shoulders.
Your shoulders are a combination of optimal movement and strength in your thoracic spine (midback), scapulae (shoulder blades), and the ball and socket joints that you recognise as your shoulder.
Stiffness in your thoracic spine is very common due to our slumped lifestyles and tight chest muscles. All of the nerves that are integral for proper functioning of your shoulder come from your cervical spine (neck) and thoracic spine. Often by getting your spine moving freer, you will notice a significant improvement in your shoulder range of motion and strength. So start by regularly stretching over the back of a chair or foam roller.
There are 17 muscles that attach to your scapula, which is floating on top of your ribs at the back. If there are imbalances or weakness within these muscles, they will pull your glenohumeral joint (shoulder) into a poor position. This will limit the range and strength available to your shoulder. Sitting and standing up taller with your shoulder blades resting flat against your rib cage will optimise the position of your scapula and strength of your shoulders.
Finally, there are some major tendons that connect your scapula and arm muscles to the bones in your shoulder. If these tendons are getting pinched at any point they will start to fray and weaken, just like a rope. Band strengthening your rotator cuff muscles and lats will optimise your shoulder position and increase general power in your shoulders.