Hello man and welcome to the forum.
It really depends your height. If u are 1,75+ u are pretty low weight.
It also depends on ur goals. Usually men like to be heavier except if they do sports that being lighter helps like climbing,gymnastics etc.
Being in about 5-10kg +- your height usually is normal.
Neither being too light (means u lack both muscles and fat) or being too heavy (especially if its mostly fat) is good for ur health.
So yeah dependin on ur height i propably would suggest gaining some weight propertly so most of it be muscle.
As for what u must do to increase weight its pretty simple!!!
You just needs to eat more!! More of what u burn!
But also try to eat as healthy as possible. If u got fast metaboslism which is propably ur case u can try to eat foods high in calories like peanutbutter,honey etc etc.
So if u dont eat more than u burn no matter what u wotn gain weight.
Now as u gain weight u want to workout , mostly strength based, maybe less cardio as it tends to burn a lot of calories. So this way the weight u ll gain will be mostly muscles and not fat. (remember u must also eat pretty much healthy and dont eat only sweets and pizzas cause of big calorie intake, it maybe work but u cant call that healthy... eating mostly shiits...)
Hope it helps!!
Goals: Be able to do bar-barians requirements in less than 10 mins, 10 good form one hand push ups, 15 pistols, Be able to do one hand holds on bar and controlled negatives, 1 min hanging l-sit, backlever hold for 5+ secs, advanced tucked FL for 5+ secs, advanced tucked planche for 8+secs etc etc
Long Goal (till summer): My 1st OAC,good BL hold, FL hold, bent arm planche, free handstand push ups, l-sit to handstand on handles, v-sit hold on floor