If your boss is an understanding person (trust me, most of them are) they will put you at another position in the restuarant where you're more comfortable. I recently got a job at a restuarant chain here in Australia and I got put on till/general front of house on my first few shifts. When you're on cashier at my work you need to ask each customer half a dozen questions, ranging from if they're apart of our membership program of if they're aware of our donation system which if they're not, we must explain it all. First shift I got put on till, and I crumbled. The words wouldn't come out and the customer was evidently frustrated. That night I couldn't sleep save two hours. I felt sick and far more anxious than I've ever been as I had a morning shift the following day. I had no choice, I had to confront the manager and I did. I told him that I have a speech impediment that worsens with anxiety and that that I cannot do till. He was very understanding, and the manager in charge let every team leader know that I couldn't do till. I've rarely been asked to do it now and if I am I kindly remind them that I am uncomfortable and they remember that I have a speech impediment and they ask for someone else to go on instead. Be up front and be honest. It's the best route to take.