New Zealand waives travel visas for U.S. citizens. You do need to be in good health and cannot stay more than nine months. More information about traveling to New Zealand is available here.
You do need a U.S. passport that is valid for three months past your NZ departure date. You can get information about applying for a U.S. passport here.
New Zealand’s electricity supply runs at 230/240 volts and uses angled two or three pin plugs (the same as Australia). For all electronics an adapter/converter from 110 to 230 is necessary, unless the item has a multi-voltage option.