Brittle nails can be caused by sun exposure, a poor diet or the prolonged use of commercial nail hardeners. Avoid the use of hardeners containing formaldehyde, which has a drying effect on nails, and try using a waterproof coating that seals moisture in the nail and repels water and dirt. Believe it or not, a waxy lip balm can be quite effective at softening nails.
Picking and biting your nails, exposure to detergents and chemicals or general nail neglect are all causes of hangnails. Use a sharp cuticle nipper to remove existing hangnails. (A dull, cheap cuticle nipper will not allow you to get at the hangnails and may create more damage.) To help prevent hangnails altogether, use massage cuticle oil, lotion or cream at least once a day. If your cuticles are a painful mess, apply moisturizer twice daily.
Ridges on the nail are, unfortunately, mostly genetic. Though you can't change heredity, you can smooth the ridged nail surface with a buffer and buffing cream. However, if you have recently developed ridges, this could be a sign of anemia, and you should consult your doctor. Ridges may also be caused by too much pressure and pushing during your manicure. When grooming your nails, always make an effort to be gentle on them so that they will grow back normally.
White spots on the nail can be caused by a blow to the nail, applying too much pressure on the nail matrix during a manicure or too much pressure and pushing on the nail, generally. Let the spots grow out, and make an effort to be gentle when manicuring your nails since prodding beneath the cuticles, where new growth is generated, can cause these spots.
Yellowing of the nails could be caused by not using a base coat underneath your polish, or it could be a more serious nail fungus that needs to be treated. Lighten discolored nails with a whitening scrub containing a mild abrasive, or with a remover containing a lemon juice-like ingredient that bleaches out the yellow.
Unfortunately, there is no surefire way to stop biting your nails. A good way to try and stop is to get a professional manicure each week. When your nails are even and polished, you're less likely to put them in your mouth. Also, the better you feel about your hands, the less you're going to cover up your nails, and the more confident you'll appear. (Now might be the time to consider artificial nails.)