You'll usually have four or five routes, and it's worth knowing more than one in case your first choice fails. The carrier's own app is the most convenient once set up: it shows your balance, sells top-ups and add-on packs, and often has app-only deals. The snag is payment, since some apps only take local cards or wallets, though many carriers' websites accept international cards even when the app won't.
Convenience stores are the universal fallback in most of the world. Tell the cashier your number and the amount, and the credit lands directly, or buy a voucher card and enter its code with a USSD string. Supermarkets, pharmacies, and kiosks often provide the same service.
A few practical habits: save your local number somewhere accessible the day you get the SIM, learn the balance-check code (the shop can write it down for you), and note whether top-up credit differs from packages, because in many systems you first add money, then separately activate a data bundle with it.