How can I come to New Zealand ?

Contact the nearest NZ Embassy, get a tourist visa, buy a return flight, book accommodation and head over. You will have to leave on or before the finish date of your visa or risk being deported and banned from obtaining any other visas.

For visa information www.immigration.govt.nz  Try this site for all visa types.

If you are young enough you can apply for a working holiday visa.

What is The easiest and fastest way for coming to New Zealand ?

Your question is a very open-ended one.  The only easiest (and probably the fastest) way to enter NZ is to be a tourist. That'd be a limited stay visa.  Rest, Stumpy has quite easily mentioned.

Is it easy to convert my NZ tourist visa to a resident/work visa? or do I need to exit an reapply? thanks a lot

Check www.immigration.govt.nz for information. If on a tourist visa you can apply for work.

usually by plane or boat or swim...