Gain weight tips anyone?

I'm in my early 20s, no older than 25, and I'm like the size of someone in grade school. Even my cousin of 5 years younger is bigger than me, which to be fair, he's a guy. But still, with my latest the height of 134cm (4.4 ft) and 38kg (84 pounds), these senior highschoolers, my cousin's classmates (even the girls) are towering over me. And I still fit my pants and shirt from my 7th grade PE uniform, I was 13 😭