I'm under 18 and my bust is 75e. My good friends laugh at me. What should I do?

Good thing, sister. I understand you. I was embarrassed when I was running in junior high school, but now I understand that they are naked jealous of you. Ignore them. It is inevitable that women will be jealous of each other together. Just wear the right bra, and take care of your breasts, which is your lifelong wealth. I suggest you go to a professional underwear store to buy a bra, buy that kind of 2+ bra, don't bury the gift of God.