Speak with confidence even if you don't feel confident. Practice speaking this way. The more you practice the more it will become second nature.
Express your needs.
It might be hard to break out of your shell the first time, but eventually people will listen to what you have to say. Let them get used to the fact that you're speaking up and showing that you have wants, needs, and desires just like anyone else.
Learn how to deal with your frustrations.
If you keep everything bottled inside, you might end up exploding in someone's face. People won't see this as assertiveness; more likely, they'll see it as arrogance or aggression. When you're frustrated, voice your opinion as soon as you can while the situation is at hand and work out your differences together.