I used to be very thin. I got no girls and the guys all thought I was weak even though I was actually pretty healthy and strong.
Anyway, I stopped smoking cigarettes, and I started going to the gym. I went kind of crazy but I recommend you go pretty heavy but slow and reasonable.
Get yourself a basic book on bodybuilding. Basic!
Don't worry about supplements but here is the secret which made me gain 5o pounds in 5 months. Eat like you are crazy.
I don't mean big amounts at one time but if you consume 800 quality calories every hour(set an alarm).
I ate normal foods. But absolutely not chocolate or candy. This stuff will burn off too quickly.
I ate alot of chicken sandwiches with cheese and monster cheese burgers. Fried chicken, potato salad. Vegetables are important but eat those last after you ate some heavy stuff.
And finally, don't go overboard. Thin people look really cool in their clothes. And later in life you will be happy to be thin and probably increase you life span by staying thin.
But, right now you want chics and that is understandable. Put on about 15 kilos and be done with it. You won't lose it again after you gain it.
I am glad I put on weight, but right now I have a nice middle sized body and I am not sef conscious at all.
Also, get some cool clothes. You don't have to spend a lot of money but try to fashion it up a bit.
And also, getting girls is a numbers game. You said you don't "attract" the girls. How do you know? Anyway, it doesn't normally work like that. You have to be proactive and learn that if you get rejected, that is just one more rejection closer to the one(or two) that are going to say OK.
Also, you are pretty young. Girls your age are sometimes pretty shallow. They care about cars, money, popularity, etc. The wonderful thing about those kind is that they are very easy to manipulate once you finally turn the tables.
And you won't feel guilty about manipulating them. They are bitches right?
But don't treat the nice ones bad. Treat them nice and appreciate them. But, always kind of keep in mind that you could get burned by a chic that you really like. If you are prepared for it and realize that there is always another one then this won't hurt you.
Women sometimes call us pigs but sometimes they create the loathsome woman-haters by showing themselves to be unworthy of a man's sincerity. It works the other way too.
Girls your age would prefer to get the shallow glory from telling their friends who they rejected, than to just actually have a boyfriend and be getting shagged.
But later, after about age 25, if you haven't already turned the tables then the table will turn automatically.
They will be getting fat and old and leeching for a man to marry. While a man usually just gets more distinguished looking with age. And you, you will be your tall svelt self, and buddy, you will be dangerous.