To come to the Netherlands, you will require a Provisional Residence Permit (mvv). An mvv is a visa that is issued for a stay longer than 90 days. The sponsor submits an application both for an mvv and a residence permit at the same time. This is called the Entry and Residence Procedure (TEV procedure).
Not everyone needs an mvv to enter the Netherlands. Your sponsor then applies for a residence permit directly. Your sponsor can submit the application while you are not yet in the Netherlands.
Only a recognised au pair agency can submit an application on behalf of an au pair. The au pair agency is your recognised sponsor.