Products
Products - New. Each product needs a name, a selling price, and a category. Add a barcode if you scan, a cost price if you want margin reporting, and the tax rate that applies.
Categories and brands are how you find things later and how reports group them. A shop with 600 products and no categories has reports that tell it nothing.
Barcodes
If you scan at the till, put the real barcode on the product. Scanning is faster and removes the commonest error at the counter: ringing up the wrong similar-looking item.
Stock
Inventory shows what you hold. Stock moves on its own as you sell and as you receive purchases - you should rarely type a stock figure by hand.
When you do need to correct one, record it as an adjustment with a reason. "Stock was wrong" with no reason recorded is how shrinkage becomes invisible.
Batches and expiry
For goods with expiry dates - pharmacy, food, cosmetics - Batches tracks stock by batch so you can sell the oldest first and see what is about to expire. The expiry report is what stops you finding out at the counter.
Purchases
Purchases - New records what you buy from suppliers. Enter the supplier, the items and the costs. Receiving a purchase increases stock automatically, which is why it should be entered when goods arrive rather than when the invoice is paid.
A purchase entered late means the till thinks you have nothing to sell while the boxes sit in the stockroom.
Why this matters at the till
Everything above exists so the till can be fast. A cashier should scan, take money, and move on. They should not be deciding prices, looking up tax rates, or wondering whether something is in stock.