Worrying about starting my new job?

I'm 16 and have just left school. I decided I wanted to get a part time job so I don't have to keep relying on my parents so the other day I decided to hand out some CV's. The first place I went into was Halfords, who interviewed me there & then and gave me a job working on the tills. This was on Saturday and i'm due to start the job on Friday. On Sunday the manager rung and asked me to come in for till training. So I did. He put me with a girl on her till and left her to show me what to do. If people are just buying things then I know what to do, but if people want to return an item or pay with vouchers i'm clueless and no-one seemed willing to show me what to do. On Friday I will be at the till alone and all I can do is worry about it. The thought makes me feel sick as I just don't know what i'm doing. It sounds pathetic but has anyone had a similar experience/have any advice? Would be greatly appreciated.