What's Wrong With Me?

I am 15 yrs old and i am very skinny. as a baby i was quite chubby but as i grew up i havnt seemed to put on any weight. i am growing taller but just not puttin on any weight. i also have a big belly which i hate! it is like a pot belly and i cant seem to get rid of it! i do eat quite alot but most of it is junk food because i dont like eating anything else. im sick of being so skinny and want to put on weight. can sumbody tell me what is wrong with me? how do i lose the pot belly? and how do i gain weight? i am around 5ft 2 and weigh only 39 kg. please help me :'(