For ideal sitting position, find a chair that allows you to sit upright with at a right angle between the torso and legs. Your knees will ideally form right angles as well. This is all to minimize any unnecessary flexing of the joints. You also want to keep your back upright and straight to avoid the tendecy to slouch over the guitar. Let the lower bout of the guitar rest on your right leg, so that the back of the guitar can rest against the torso.
The resting point for the fingers is in between flexing a fist, and extending straight outwards as though waving your hand. It’s this in-between neutral position that you want to keep our fingers at whenever possible to avoid extra tension. By the same logic, you want to aim for both elbows to be at a right angle between the forearm and bicep. Finally, try to keep the wrist of both hands as straight as possible at any given time. This can help to avoid repeated stress injuries over time.
This can be a lot to think about at first, so just remember that these are just guidelines for an ideal posture. Don’t let it make you feel too rigid. You want to be flexible and expressive when playing music. See what feels best for you, and know that this information is here to revisit when you need it.