Nail biting is usually seen as a nervous habit (anxiety) - but this is only one explanation for why people (kids & adults) may nip at their nails.... of all the nervous habits — which include thumb sucking, hair twisting, and nose picking — nail biting is the most common one.
Please keep in mind that you may be doing this for a number of other less worrisome reasons — out of curiosity or boredom, to relieve stress, to pass the time, or simply from force of habit.

What to do - - Keep your hands occupied....
If you can pinpoint the times and places when you are most likely to bite your nails — while watching TV, for example, or riding in the car, when anxious, while feeling upset or hurt — try giving your self substitutes such as a squeezable ball, or a bendable toy / item to keep the hands busy. (and) Cut the fingernails short so there's nothing to tempt you to bite.... Good luck.

LoVe,
Rhapsody -