If you are coming to New Zealand to study for more than three months, you will need a student visa. For practical information on student visas, including the application process, see the New Zealand Now website.
A guide to filling out a student visa can be accessed here.
Education New Zealand accredits education agents in a number of countries to give students and their parents assistance in selecting and applying to New Zealand educational institutions. An agent can be a great help in reviewing your options, and assisting with student visa applications and other paperwork.
You can find the contact details of accredited New Zealand Specialist Agents here.