How can i improve my muscles etc?

Ok lately i have had a urge to fight i have not had a fight but i dont know what it is something just makes me want to fight constantly everything i think is how strong will that be what could this be i always think about beating people up i havent had a fight for like 2 years literally and am a little worried to fight now because my last fight i got tapped on the back turned round got about 10 - 15 punches to the face fell to the floor knocked out for about a few seconds i got back up ready to fight with blood pouring down my face and alot of it is it normal to get knocked out with about 15 unexpected punches or am i just weak ? then another day i was walking down the street minding my own business when this person came up to me and said arite then punched me right in the jaw it did not knock me to the floor and to be honest did`nt really punch it was just the one punch but i was ready for alot more but he ran away with his mates and he had like 5 friends with him. I am 6 foot 1 and 10 stone roughly , 14 years old , and am not fat btw im very skinny but still have alot of strengh can pick up people in my year literally and throw them down the whole corridor and there is a person at the moment thinking he can take the mick out of me i have had several fights with him every time winning him but i dont know whether i am soft. I have never been amazing in fighting but can definatly pack one hell of a punch not being big headed put believe me i got whipped in the face once punched the kid twice in the both arms he had huge brusises and was crying on the floor but believe me being whipped in the face aint the nicest of things and he did do it on purpose also , what is a good age to start buliding muscles without stunting my growth ? because i would love to turn my skinny self into a muscly self :) lol would be nice to take my top of and not see my little belly sticking out lol and also .......................... thanks guys for taking the time and effort to answer much apreciated ! thanks guys