I used to be exactly the same. When I was at school/college, I was the quietest person ever, and would hate having do stand up in front of people to do anything like that
However, in my last year at college for my Law A Level I had to do a presentation on a topic of my choice, I was dreading it but when it came time to do it, I really loved it, and got a really good mark. That was it then, I just changed completely, it was like Id conquered my fear!! Ever since then speaking in public has never fazed me, and my confidence has really increased!
I now help to running training courses at my place of work believe it or not so standing up in front of people and giving presentations is a regular occurence!!
You just need to forget about everyone else, just go for it and enjoy it. Once you've done it a few times your confidence will really start to improve!!