I belong to a church but I am a shy and nervous person,?

Basically I am terrified at having to undergo any public speaking but I know one day i will be expected to, I dont have a problem with small groups of people but speaking in front of large groups worries me enormously ,I am not a natural smiler or laugher which doesnt help , This is not just specific to church but in any other areas of life. The thing about it is that i can imagine myself doing well if i tried but i always bottle it when the opportunity comes . How can i overcome my fears ? does anyone else go through this?