How to gain weight

i always tend to google this, i just really need help ! 
im 19 yrs old, 5'1 in height and i weigh about 83-87lbs
i eat alot and no matter how much i eat, i never gain weight. 
If anything im always losing weight. 
How can i gain weight, because i feel soo insecure. i feel like im too skinny and my clothes never fit me rigjt, they always fit me big