Have you used any dark nail varnish without using a basecoat? That too can cause yellowing on the nails.
Liver problems like terin400 mentioned, smoking (which I doubt that you smoke), some medications as well as an unhealthy diet lacking in Vitamin A, B, zinc, protein, calcium and iron.
You may benefit from a multivitamin supplement suitable for your age group. But make sure that you eat a healthy and balanced diet consisting of meat, milk, cheese, yogurt, fresh fruit, vegetables and nuts. Brazil nuts contain zinc Cut out all the junk foods, fizzy drinks, sweets, cakes, biscuits, chocolate. Eat these as a treat occasionally.
Don't forget that your bones are still growing and you need a good supply of calcium and Vitamin D that helps to absorb the calcium in your body. If you eat a healthy and balanced diet you should be fine.
You could make an appointment with your doctor to check out your liver function as mentioned by terin400 and your iron levels. This is simply done by a taking a very small amount of blood for testing at the laboratory.
Best wishes.
A zinc deficiency can cause white or light yellow spots on the fingernails.
Have you been peeling any carrots or onions?
Yellow fingernails can also be from a fungal infection.
Have your doctor check your liver function I know when babies are born and are yellow its jaundice