A common problem with guitarists is stiffness that is often a result of holding the guitar improperly. Angle the neck of the guitar up, to a thirty-degree angle. Hold the fretting hand out in front, palm facing up, and curl the fingers toward you. Bring the hand back to the guitar, with the thumb behind the neck. Let this be a natural pose, without a bend in the wrist. A straight wrist will make it easier to maneuver around the neck.
Here's few more ideas that may help: Play while standing. This will force you to hold the guitar correctly. As the feel becomes more comfortable, attempt to sit and play. Hands and arms should be very loose and relaxed. Try not to tense the shoulder or bicep of the picking hand.
With light pressure, hold the pick between your thumb and first finger.
BREATHE…when trying out new or complex music. May seem simple, but breathing is important for maintaining relaxed muscles.
Hope this helps
Yours in Music