how will I ask a customer please step over to the next register please?
Hi SweetRose. Welcome to the SpanishDict forum.
I would say "Por favor, vaya a la siguiente caja (registradora),"
You can use the word "registradora" but many times people just say "caja" and everyone knows what you mean.
Also, I believe a cashier is a cajero (male) or cajera (female). So, an assistant cashier would be...well...I'm not sure if it would be cajera asistente or asistente cajera or asistente de cajera or what.