Go to Sales, then Products and inventory. Add products with a price, sizes or colors, tax, a SKU or barcode, how many you have, and when to warn you that you're running low. You can also note who you buy each from and what it costs you. Stock goes down as you sell, and you can count it any time.
Or choose Draft products for me: we draft your list from what your site already says, a price list you paste, or a photo of your price sheet or labels. You check every product, its price, and the tax, and nothing is added until you choose Add the ticked products.