How do i gain more weight?

im eleven i should be 76 pounds atleast the doctor said but i think i should be a little more i am seventy pounds though so i need to gain six punds not that much actually but im supposed to be going through a lot of puberty but im not really so i crave on junk food and that makes me Gain weight but u m supposed to eat healthy food when i do that i gain no weight at all how do i stay healthy but gain weight or shud i keep eating junk its no bother i think atn this time I need it please help meeeee gain more like 96 punda weight