How do I fix my reputation?

I am in high school right now. Since I had some problems with hygiene in middle school, girls think that I am disgusting and greasy and stuff. I am very hygienic now, but people still see me in this kind of way. This is really embarring, plus it ruins my high school life, such as when I ask a girl to Homecoming or Prom. I am also slightly overweight. How do I change the way the people think of me? I am a Junior, and I am running out of time! I don't want to go to prom by myself. Please help and be detailed, don't say stuff like "turn your back on the world" or "you shouldn't care", because I can't turn my back and I DO care.