This is a question I get asked a lot by those starting to learn to draw. Holding a pencil for drawing is similar in a way to holding a pen for handwriting in that there are ways that get better results, however it is a personal choice. Just like handwriting we all have a unique style so I think the best advice is however it feels best.
Having said that there are some general points to consider. The pencil should not be held really low down and gripped too hard. This won’t allow for any movement in the line or flow in the drawing process.
It is considered that the higher you hold the pencil, the more movement you can get in the line and a more fluid, loose feel will result in drawing. Many beginners find this difficult at first, holding the pencil like this comes with gained confidence naturally when you draw more often.
The overhand grip is often advised as the desired way to hold a pencil.
Although I have read some art books that advise certain ways to hold the pencil I am a believer in doing what works for the individual. The most important thing is that your hand is relaxed when you are drawing and that it feels comfortable.
It is sometimes a good idea to change the type of grip you have on the pencil for different types of strokes. I like to hold the pencil in several different ways to create different marks. Try a few different ways of holding the pencil and see what works best for you.
Also see Pencils Explained for Beginners