I'm not assertive enough to stand up for my friends, help?

Okay quick background, I'm a runner/bodybuilder, the whole school (except myself oddly) says that i'm the most ripped and in shape guy in town. If anything were to come to an end with some sort of fight, i would easily pummel 99% of the guys at my school.





but i don't care about that, i'm like a gentle giant. I can't force myself to yell at someone, or to stand up for what's right, or even step in if someones making fun of one of my friends.





i can't assert myself at all, and people are starting to see it.... i've seen guys get away with cheating on my friends who are girls, and i couldn't force myself to do anything to stop it. I've seen a good friend of mine get into a fist fight with a kid twice his strength who he didn't stand a chance against, but for some reason i wouldn't step in and help him.





i want to be able to use my strength for more than just ';showing off'; and ';looking ripped';. I'm not saying i'm looking to pick a fight, but how can i get myself to stand up for the little guy every once in a while?I'm not assertive enough to stand up for my friends, help?
Aww you need to help out the little people