Nothing's "wrong" with you,you're probably just a little shy,like so many others in this world There are 2 types of people,those who start a conversion and those who wait to be approched first...what if we would all start talking at the same time,it would be a mess xD I'm sure you'll built up your confidence with time, when you're approached just sway along with it and everything will run smooth.
That doesn't quite tell me why you get nervous. I'm thinking insecurity. You know, you're worried about sounding stupid or something.
Insofar as that's true, my advice is to intentionally say stupid shit to people. Not at a job interview or anything, but if you don't have to worry about consequences too much just say inane things and you'll realize that you can survive doing so. Once you're over that hump, general chatting might come more naturally to you.