Posture is not complicated

Promise me you won't over think it too much. The big issues we will gradually discuss over time as we get more comfortable, but just know the only 'perfect' starting point is that where you are just comfortable while holding the instrument.

If you're getting any discomfort at any part of the arms, neck, lower back or shoulders, then it's almost always related to how you're sitting with the guitar and in my experience, raising the leg that the guitar is resting on cures most playing related issues.

If you're interested in getting a footstool, here is the one I use. (I am not an affiliate for any products I recommend you.)

Below are some special considerations for sitting with different guitars

Go to any guitar forum or group and watch how people sit and hold the instrument. Personal enjoyment is one thing, but if you want to enjoy the instrument, discomfort free, it really doesn't take much awareness to get it right - just a few simple steps. Don't overlook these details.