Bro, I'm the same age as you, studying in JC right now too.
Beginning of this year I could barely do one or if I was lucky,
two pull-ups (I've never been/I'm not in a sports CCA)
Right now, I can do ten or eleven.
As kopiosatu said, the trick to pull-ups is simple: keeping
doing pull-ups.
If you can't do one, just keep doing assisted (get your
friends to support you while you try to pull yourself up with your
own arm strength, but NOT them doing the work and pushing you
up) AND declined pull ups (jumping all the way up, getting your
chin above the bar, and then slowly lowering yourself while
resisting gravity with your arm strength).
As others have suggested, doing weights/push-ups to build up
your arm strength might help to get you started on your first few
unassisted pull-ups
Do note that while pull-ups (also note the difference between
pull-ups and chin-ups)require you to have substantial upper body
strength, pull-ups are still mostly utilizing your back/wing
muscles (if you do it "properly") and your biceps.
Depending on the grip you ultilize and width of your arms during
your pull-ups, huge forearms/triceps doesn't automatically mean you
can do loads of pull-ups. Also, just because someone can do 100
push-ups in one go, doesn't necessarily mean they will be kickass
at pull-ups too.
Don't bother using those gym machine assisted pullup machines,
just train the good-old fashioned way. Trust me.
Let me put your mind at ease, including for your mentioned
weight/height concern, don't worry.
Beginning of this year, I was 177cm and 59kg. I couldn't do
more than 10 pushups and 2 pull-ups without dying.
Now I am still 177cm, but 65kg instead. I can do 40 pushups
straight and 11 pull-ups no problem.
Mind you, I improved WITHOUT gym, WITHOUT taking up a sports
CCA, WITHOUT any protein shakes and whatnots, and with a
ridiculously high metabolism rate (I'm one of those typical
eat-a-shit-load-never-grow-fat-or-muscular sort of people).
Keep going at it bro, good luck.