The fact that you have written to me directly instead of through an older person is already evidence that you have courage to handle your own issues. Well done!
Being brave is mostly in the mind. You should convince yourself that you are strong and courageous.. When we watch karate shows, we find that mind power helps people enormously in putting up with pain and carrying heavy weights.
If we decide that there is nothing to be afraid of, we are able to be brave. When we fear every blowing wind and shaking of a branch to be loaded with danger, we anticipate something terrible and see things which are not there. Your own resolve is therefore very important.
But one should have realistic fears of sharp weapons, poison or fast traffic on the road. It is sensible to be afraid or to have fear in such cases.
Perhaps your question relates to dealing with other boys of your age, who may be bullying you. Bullies are often just cowards. Faced with a real threat, they will disappear. If you are not out-numbered, you can tell the boy that you will not stand for being bullied and that you can also take the offensive.
You should be active in games and sports and physical exercises and build up your strength, so that the other boy will not tease you so easily. A little bit of healthy fighting is what all children go through. Instead of always being the good guy you may just show them that you can defend yourself and attack, if provoked. There are many adventure stories in print. If you read them, you realize that hundreds of people, young and old, have achieved tremendous feats of bravery and endurance.