17 years old weighing a little less than 100

i’m 17 years old and i’m SOOOOOO skinny! at times i love my body and how petite i am. But at other times i feel so uncomfortable and just hate my body. I have a fast metabolism so its really hard for me to gain weight but SOOO fast for me to lose any weight i ever gain. I dont wanna hear anyone judge me for trying to gain weight, so i’m asking kindly for any advice on how i can gain weight? I wanna gain weight in my legs, butt, and arms because they’re soo scrawny. What are some foods to help gain weight? Any protein shakes? Also if i join the gym what are some ways I would be able to gain weight and not lose weight because my friend has been going to the gym and she said she went from a 32A to a 30AA or something like that and i dont wanna lose weight just gain