If you are going to study in New Zealand for 12 weeks or less, you do not need to get a student visa. You can apply for a visitor's visa instead.
If you are going to study in New Zealand for more than 12 weeks, you will need to apply for a student visa. Full details of immigration requirements, advice on rights to employment in New Zealand while studying and reporting requirements are available from Immigration New Zealand. You can also apply for your NZ student visa online through the Immigration New Zealand website.