How to reduce back pain while standing at work at restaurant?

Get better shoes. If this is a new job, you probably need time to adjust. It is a good idea, however, to look at your shoes. Shoes that are well made with good arch support will go a long way to helping your back pain.
Move around more. Our spines do not react well to staying in one position too long. This tends to cause the soft tissues, (discs, ligaments, muscles) to deform which cause symptoms. Best to move around. If your back still does not respond, you might do well to work with a physical therapist to strengthen your back.