How to be a better sales person?

I work at a teen clothing store and I still get nervous talking to people, is there a way I can talk and sell somthing to someone without feeling like fainting or talking so softly. I have a loud voice, it's just nerve wracking when your working with your store manager and they're watching every move you make and tell you hey you did't do very well on this or you could've done this when you done everything you could possible do