Problems with my nails

I am a girl of 18 years. I recently noticed that my fingernails are turning brown and the nails on my toes are cutting into two. Kindly let me know the cause of these and what I should do about it.

Blessing (by SMS)

The most common causes of yellow or brown fingernails are the frequent use of nail polish and cigarette smoking. Taking a break on the polish or quitting smoking may allow the dark nail to grow out as the new nail comes in.

A yellowish colouring of the nails may also be caused by diabetes, lung problems, arthritis, lupus, certain medications, lymphatic system disorders and liver disease. Not having enough vitamin B-12 can cause nails to turn dark, thin and possibly spoon-shaped

Fungus infections of the fingernails can also turn the nails brown or yellow. The texture usually also changes; nails often become very thick and may crack or peel. Nail fungus infections are more common in the toes than the fingers.  These infections can be difficult to treat, and taking an oral antifungal medication is usually required. However, if things do not improve, you may need to see your doctor for a more expert advice and treatment