I am a 16 yr old girl who wants to gain weight?

Helloo i would like to kno how to gain weightt because i feel as if im too skinny andd by gainning weight i want to have bigger also..i am 5'6 and 120 pounds,it is also very hard for me to gain weight. iIwould like to at least weigh about 135 is there any way i can gain that weight?