Please help this teenager calm down? ?

Alright I'm 17 about to be a senior, and I play baseball. Its always been fun for me, but as of late I've been getting kind of nervous before games, and we have a game against a really good team and my coach told me I'm starting. And I'm really nervous. My stomachs somewhat upset and I'm really antsy and what not. I don't know what to do. I try to block out the fact its a big game but I can't, and I know this nervousness will make me play bad. What can I do or take to get myself to calm down.