What makes you hit harder/have a harder punch? [more impact]?

Im not good at ANY sports really.. but i do love boxing and i want to be a boxer more than anything in the world. i dont really look like one.. 5'4'' shortish blonde hair.. whatever. but i feel like i fight way better when something is there to piss me off. like if a girl was tryin to be all over my boyfriend.. id beat her AzZ!! but if i was boxing.. thers nothing really there to set me off other than the fact that you either beat or be beaten. i have a pretty hard hit, or so ive been told.. people are always like "you hit hard for a girl" but i know that sometime, someone is gona be better than me so i want to work on that, coz i feel like it could be so much harder.. Does any one know of any excercise i could do that would improve the impact of my punch?? i dont know what its called but i think if you work out the muscle in your armpit/chest that'd help-- but what do i know?!?! please help me!