You don't need a calculator.
The first part is always the fun of how to sign on- differs if it's your own till that no-one else uses or if it's a shared till that you just log onto. Then how to swipe a loyalty card if there is one. Then the till will show the normal sales screen. There'll be a keypad of some description if you need to type in a barcode for whatever reason. So you scan stuff blah blah blah and press subtotal. Then you have to be trained on various methods of payment. Cash, credit cards, debit cards, how to enter cashback if there's the option, how to process cheques if you take them, how to do giftcards/gift vouchers/coupons/student discount. Then there's stuff like refunds/exchanges if you're entitled to do them, voids- cancelling an item/the whole transaction, overriding a price if it scans in wrong...and that's about it. Do not make a load of change for large notes for someone or make more change for someone after already dividing their own change. It's a scam and you will lose money.
It's actually really easy.