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.