. Rome wasn't built overnight and nor do you have to overcome shyness in a day. It takes time, but you'll get there.
2. Stop focusing on yourself
. Instead of thinking "oh god oh god oh god" the next time someone addresses you or pays you attention, shift your focus from yourself to that person. In what way can you help him or her? This eases the tension of thinking that you're taking up that person's time or don't have anything viable to say.
3. Be yourself
. Ok - a complete cliche, but as soon as you realize that you have a great personality and people actually WANT to talk to you and are interested in what you have to say, it helps with the tension.
Hang in there, you'll find your voice.
I struggled with this for years (I'm definitely not yet completely confident, but I have made major progress!)