I am 13 under weight and have the most flabby stomach out of all my friends. I need to loose the flab! ideas?

Some say I breath with my diafram too much and thats what i did all my l.ife and that I stretched my belly they say I need to breath lifting my sholders. My mom says that I am just carrieing baby fat and it will go away soon but nothing! I can inhale and exhale extremily far one minite I look like I'm pregnant and another minite I am almost perfect but I cant suck in my whole life. I need to get rid of this gross stuff but how?