Most visitors need an Electronic Travel Authorization (ETA) before arrival, applied for online. It is not a visa-on-arrival system for most passports, even though it is sometimes described that way.
Apply only through the official government portal. a number of lookalike third-party sites charge a markup for the same thing. Approval is usually fast, often within 24 hours, but applying at least a few days ahead avoids any risk around a slow queue.
The standard tourist ETA covers a 30-day stay and is extendable. Bring a printed or saved copy of your approval — some airline check-in desks ask to see it before boarding, not just on arrival.