everyone acts differently with different people, that's not odd at all. And everyone changes a bit with time too. You don't have to suddenly become a new person, but if you do want to come out of your shell a bit more, just do it one step at a time. Join in the odd conversation; or ask someone how their holiday was or what they think of Lost. Start by listening more than talking, as Ice suggests. (Nothing impresses people more than the thought that someone is listening to them!) In another year you could be a completely different person without anyone actually realising how you'd changed.
I'd also say, actually, don't worry, the world is full of quiet types and nobody respects them any the less, it's not something disgraceful to be. But you seem unhappy about it, so in that case it's worth trying to change... but gradually.