There will be plenty of signs with instructions when you arrive at your airline terminal. Follow the ones that say "Arrivals" and you will eventually get into a line for non-residents. Once you reach immigration control officials, you will show your passport (which contains your visa), landing card and certification letter. Officials may ask a few questions, but nothing daunting.
After the official stamps your passport, you'll head for the baggage claim carousel. Check on the overhead monitors to see where your plane's baggage has been unloaded.
Finally, you'll move on to customs. At customs, you'll head for the "nothing to declare" channel unless you are carrying goods that exceed the duty free limit.
IMPORTANT: New Zealand's economy is very dependent on agricultural and horticultural produce, thus it's extremely important that harmful insects and diseases are not inadvertently brought into the country. As a precautionary measure, you may be approached by an officer with a sniffer dog and required to complete a form asking you to declare any foodstuffs and/or plant/animal products you may be carrying and any recent contact with farm animals. Generally, you can bring in commercially packaged confectionery, but it is advisable to come with as little as possible.