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.
It is possible to rent vehicles at least in Ribeira Brava and Tarrafal. Preferably rent a vehicle with 4 wheel drive. Most of the roads in São Nicolau are unpaved and are in poor condition. In addition, the island has several zones with considerable altitude and, in these zones, considerably steep roads.