Keep your nails very short so there's less chance of breaking one and being tempted to nibble a piece off. Maybe have a professional tidy-up, so you won't want to waste the money by biting the results. Sit on your hands if they start to wander to your mouth. As soon as your nails look more decent, use some nail varnish, you'll be pleased to have attractive nails and want to keep them that way. Once you make the effort to stop biting, it shouldn't take too long before you see results.