Surgery, jelqing, a penis pump or losing some weight are some options. I don't recommend surgery. If you have the time and patience you could do some jelqing exercises, google it if you don't know what it is.
By the way you are 13years old, so your penis would still grow till you are 15 or 16 years old (in erect size). Not till you are 21. Your flaccid/limp size could grow till you are 21. I don't recommend jelqing till the age of 16, because you might hurt yourself. If you are fat, you could lose some weight, which also could help.