The
fastest way to build courage is to do whatever you fear and look it right in the eyes. Hate being in front of crowds? Then that's what you must do.
There are other really quick methods you can do (such as dressing nicer, standing taller, talking more sternly, and so on), but those only mask the problem. Eventually that veneer of 'courage' breaks when you're confronted with something that really scares the hell out of you.
Face what you fear, and it'll go away.
I used to be super shy, to the point where I didn't even want to order anything from the menu at fast food places because I
hated to talk to the cashier. Fast forward five years later, and my #1 favorite thing to do on earth is perform on the piano in front of crowds and give shows. It's such a rush, it's exciting, I love every minute I'm on stage performing! I could never have closed that gap had I not buckled down and started tackling my fears and building courage through serious means.
Get to what makes you fearful, tackle it down, and your courage will grow! It's really simple to say, but hard to put into practice.