fly with the TACV, from Praia, Mindelo and Boavista to the Preguiça aerodrome, in São Nicolau, for starting at €100.
You can also reach São Nicolau from Mindelo (São Vicente) aboard the ferry Liberdadi that connects the two islands twice a week. The trip takes about 2h 45.
There is also the alternative of arriving in São Nicolau from the city of Praia, aboard the ferry Kriola. More information at FastFerry CV.