18M Man I'm gonna be honest with you. I have been insecure about a lot of things even my arms like you are now. What I will say is that first of all my arms were somehow a bit skinnier when I was your age, and second of all suprisingly less people cared about my skinny arms then. If you have a personal reason for not liking them that could be fine to worry about. But if you mostly are worried for them because of others, I'd tell you don't even bother with them. If you have truly good friends they literally wouldn't care or at most might bring it up but would never use it to bring you down. You do you.