I can understand your concern.
I have calculated your bmi and it comes to around 16 which is less. But still you are 14 only. No need to do anything as such. Just have frequent meals and take a high protein and a high fiber diet.
It will increase little by little.
With your age, your weight will also increase.
Best regards. Dr. Anil kumar jain.
As long as you are active, have adequate energy to go through the day to day activity, you should not worry too much about your weight, however sometimes failure to gain weight may be due to hyperactive thyroid, so please get your thyroid function checked once.
Please increase in take of proteins in your diet, that is egg white, chicken, fish. If you are vegetarian then have plenty of almonds, whey protein powder, peanuts and broccoli.
Please join gym and start doing work out, you will gain muscle mass by excising, that will result in increased weight.