If you are going to work the point of sale, here are the few things you will need right away. You will learn the rest quickly as you go.
The most important things to master
Overview of the point of sale screen — what the buttons do, and where you find items.
Create a sale — add items and complete payment.
Returns — take an item back and handle store credit.
Discount — give a discount correctly on the sale.
End-of-day — close the checkout at the end of the day so the figures balance.
Get off to a good start
Point of Sale is built to be fast in a busy day. Spend a little time trying the five things above in peace, so they stick when there is a queue.
See the concrete how-tos in New sale, Menu buttons at the point of sale, End-of-day and Point of Sale (quickguides).