The sitting position is best to use when you are first learning to play the guitar.
When playing sitting down:
The standing position is particularly good for playing electric guitar (but can also be used with an acoustic guitar) and is essential if you plan to play in a band. Once you are comfortable with this position, try moving in time with the music as you play.
When playing standing up:
The correct position for the right arm is illustrated in the photos. Notice that the forearm rests on the upper edge of the guitar, just below the elbow. Be careful not to have the elbow hanging over the face of the guitar or your hand too far along the fretboard.
When you are playing single notes, support your right hand by placing the second and third fingers on the body of the guitar. If you are playing an acoustic guitar, pick the strings over the sound hole as this makes the best sound.
If you are playing an electric guitar, pick the strings between the pickups, as shown:
If you are strumming a chord, do not support your right hand with your fingers on the body of the guitar but still rest your forearm on the upper edge of the guitar.
The correct placement of the left hand is illustrated in photos A and C. Be careful not to allow the thumb to hang too far over the top of the neck (photo B), or to let it run parallel along the back of the neck (photo D).
When playing notes and chords your fingers should be on their tips and placed just left of the frets (not on top of them) as shown in the picture.