How can I get over my shyness in order to perform my job well?

I am about to start working at a hospital as a volunteer. I will most likely be doing paperwork or helping people find their way to various parts of the hospital. I am a moderately shy person, and sometimes I stumble over my words and end up looking like an idiot because I'm intimidated by strangers. I don't want to be like this because I want to help in whatever way I can.