You could go to the gym and do some bench presses and machines that help. If you are underaged i would advise you to do pushups, pullups (Buy a pullup bar) or dips!Good Luck!
You should compare it to your triceps ... the biceps should compose 1/3 of your arm size for balance. If not, you need to relook at your training.
a lot bigger than yours.
Biceps are 18+ inches
the weight lifting machines