How to gain more weight and be more confident?

I am 13 years old. I am about 5'8 and I weigh only like 100. I've always been skinny like that. Behind all the bones are weak muscle, but I see it. There is a lot of muscle in me but I just need more meat on the bones. My dad is huge muscle wise and he is always getting on me because of my build and it gets me very agrivated. Throughout my years at school, I really never wear T-Shirts. I only wear like sweatshirts and thats it and alot of ppl notice. My dad is going to get me member ship to his gym and I used to go there alot, but for some reason I just don't go anymore. I know that deep down if I want to stop being skinny and hating myself for it, then all I need to do is go to the gym, but for some reason I just don't go. I am thinking of starting over and sucking it up, and take a couple of hours out of my day every single day and start getting muscular, plus, I am going to get a new diet and force myself to eat more, I have never been one to eat much. What are any tips or help to get me back into shape? What should my diet consist of? And what kind of exercises should I be more focusing on? (I am pretty bony every where and I want to cover that up with muscle) Thanks for your time and help.