How to Hold a Guitar Correctly: Posture and Hand Position
Posture doesn't affect the sound directly, but it does decide whether you can practice for 20 minutes without back or wrist pain - which is exactly why it's worth learning first, before your first chord.
Sitting or standing: where to start
Beginners find it easier to practice sitting down - the guitar's body can rest stably on a leg without needing to be held up by a strap or shoulder muscles. Standing (usually with a strap) is worth trying later, once the basic hand motions are already familiar and don't need conscious control.
Which leg to rest the guitar on
The classical position (more common for acoustic) rests the guitar on the left leg with the neck angled slightly upward; the position more common for rock and pop styles rests it on the right leg with the neck close to horizontal. Both are valid - what matters is that the body sits stably on its own, without constantly being held up by an arm.
Neck angle and height
The neck shouldn't droop downward - too low a position forces the fretting wrist into an awkward angle to reach the frets. A raised neck (roughly parallel to the floor or slightly above) gives the wrist a more straight, natural position.
The fretting hand: thumb behind the neck
The fretting hand's thumb should rest on the back of the neck, roughly opposite the index finger, rather than wrapping over the top like gripping a steering wheel. That position gives the other fingers room to stand more vertically over the frets - which directly affects how clean chords sound (see the dedicated article).
The picking hand: elbow free, not pinned to the body
The picking hand should move from the forearm and wrist, not just the hand itself - if the elbow is pinned tightly against the guitar's body, the motion ends up stiff and small. A small gap between the elbow and the guitar's body gives the arm room for a natural motion while strumming or picking.
Try it in practice
Open the metronome