How to get rid of armpit fat?

I'm a teenage girl. As a kid, I was overweight and lost nearly 40 lbs the healthy way. I never toned up though, I just lost fat. Anyway, I've had armpit fat before and after I was overweight. I can't wear anything strapless. I want to get rid of the armpit fat (not my boobs though). I looked it up and people were like "its breast tissue" and "get a bigger size" but the armpit fat is there even with no clothes on. How do I get rid of this fat? Ive joined a gym and am about to get fit. I want to firm up my arms but keep my boobs. I know spot training is impossible. What should I do?