Why am I so SHY!?!?!?

I'm fine when I'm with my friends, in fact I'm one of the noisier ones! But whenever I'm somewhere where I don't know anyone, I just CAN'T talk to anyone. I have no idea what to say, and if someone starts up a conversation with me I panic and spout a load of rubbish and then they just go off thinking I'm weird. I can't talk to people I don't know on the phone either, for example if I'm phoning up to ask about something I just can't do it! I usually say "sorry, wrong number" and put the phone down. WHY am I so STUPID about that sort of thing?