how can get a taxi from airport to Yiwu and how to reserve it.

Well, it's not necessary for you to book the taxi in advance. You can find many taxis and private cars outside the airport. But it's expensive to travel that far by taxi, about CNY600.
Or there is also another cheaper way for you to choose. Take direct bus at the airport to Yiwu.
The bus schedules are from 10:00 to 22:40 with the interval of 40 mins. It takes around 2 hours on the way. And the ticket fare is CNY 62 per person.