Help on gaining weight?

ok....well i'm 5'2 and weight 87lbs...i have always been extremely thin....i have been checked by plenty doctors and they cant find anything wrong with me...they say its hereditary...but i hate being so petite...i eat so much but cant seem to gain any weight...is there anyone out there who can give me some tips or advice??? my goal is to "at least" reach 100 by December..if thats even possible for me hehe..i would really appreciate the help..it can be anything from what to eat..high calorie foods...exercise...anything really i just want to gain weight and fast