You can either sit or stand while playing the guitar, and the position you choose makes virtually no difference whatsoever to your tone or technique. This really is just a matter of personal preference and it's totally up to you which you choose. You may however find it easier to sit down when you're starting off as you will find it easier to look at the frets and coordinate your fingers. Most people prefer to practice while sitting but perform publicly while standing. The one exception to the sit or stand option is the classical guitar, which you normally play in a sitting position.
HOW TO HOLD THE GUITAR
A Beginner’s Guide To Playing Your Guitar
To hold the guitar in a sitting position, rest the waist of the guitar on your right leg. (The waist is the indented part between the guitar’s upper and lower bouts, which are the protruding curved parts that look like shoulders and hips.) Place your feet slightly apart. Balance the guitar by lightly resting your right forearm on the bass bout. Don’t use the left hand to support the neck. You should be able to take your left hand completely off the fretboard without the guitar dipping toward the floor.
Look in the photo below as Tommy Emmanuel holds the guitar.
To stand and play the guitar, you need a strap that is securely fastened to both strap pins on the guitar (or otherwise tied to the guitar). Then you can stand in a normal way and check out how cool you look in the mirror with that guitar slung over your shoulders. You may need to adjust the strap to get the guitar at a comfortable playing height. Look again Tommy
Classical guitar technique, on the other hand, requires you to hold the instrument on your left leg, not on your right. This position puts the center of the guitar closer to the center of your body, making the instrument easier to play, especially with the left hand, because you can better execute the difficult fingerings of the classical-guitar music in that position.
You must also elevate the classical guitar, which you can do either by raising the left leg with a specially made guitar foot stool.