If an idea is rightfully yours then you should ofcourse speak for it. If you feel scared initially then practice what you're going to stay beforehand. You'll be more confident if you're already prepared for what you're about to face.

Start by standing your ground in front of your colleagues. Once you've gained respect amongst your peers, then approach your boss. You'd just have to fake confidence till you can actually make it.

I have experienced this, so basically these people called managers there are two kinds of managers. One who does all jobs and gives courtesy to his peers. Other one gets idea from you and get the appreciation. Talking about the point where you say you are shy. It would be hard for you to speak out things, but in an organization where your ideas are being stolen by your boss or co-workers then you should find your way and pack your bags. But just so you know these things always happen in all organizations people who steal or whitelabel others work. And just be sure when you say shy, you are not asking someones hand to marry or proposing a girl. You are just expressing yourself for that you don't need to feel shy, confident is something that you will get gradually after building some courage and talking things with managers or boss.