It is important to know not only how to hold the guitar pick but also how to use the guitar pick. First of all though, why should you use a pick or plectrum?
The main reason why you should consider using a pick is because of the unique, percussive sound you can extract from the guitar. Another reasons to use a pick is that it is easier to be more accurate and to play more quickly when playing individual notes.
If you are playing with a pick and you are not holding it correctly, you can end up playing the guitar with your hand bent and too far from the guitar. This places limitations on the use of the palm of the hand for special percussive effects.
You can also get a few more sounds out of the guitar using the hard edge of the pick to get higher frequencies.
The reason why some people do not use a pick or plectrum is because they do not know how to use it. There is a technique to holding the pick.
Start off using a very flexible pick. Get one that isn’t too slippery and easy to grip.
Hold the pick between the thumb and index finger. The thumb goes along the pick covering the center and beyond the edge of the pick. The index finger pointing down and gripping with the side of the finger rather than the flat part. The other fingers should fan out and be relaxed.
Lie your hand down on the guitar, nice and relaxed, fingers pointing down. Looking down at your thumb, there should be an imaginary line between your thumb, your wrist and your elbow. If your wrist is on the guitar and your pick is touching the string and your wrist is hovering just above the strings then you are in the right place.
This might feel unnatural to begin with but like anything you have to practice until it becomes second nature. Just remember not to grip the pick too tightly as this will slow down your playing. Hold the pick firmly enough that you will not lose it, but relaxed enough so you do not feel tension in your arms and hands.