Some advice for you if you are just getting started playing guitar..
You need to be sitting with your back straight (always avoid slouching or leaning back).
The body of almost every guitar is made with a curve designed to fit on your leg when you are sitting down.
Aim to spend 10 minutes a day practicing ..
This is extremely important in order to remember what you've learned and to build
strength in your hands and wrists.
Keep your fingernails short
(Most importantly on the left hand) If you don’t, then your fingernails, as they get longer, will stop you from holding the strings down properly on the fretboard and they may even get pushed back into your fingers or they could damage the soft wood of the fretboard.
When you are learning and concentrating on what you are doing, it's natural that you will become tense, your neck and shoulder muscles will tighten up and you will feel uncomfortable. It's important to be aware of this and relax your shoulders and neck when practicing.