All travellers require a visa. A yellow-fever certificate is required if coming from a yellow-fever country.
Visas are necessary and a significant headache to obtain. There is an online system (www.smf.st/virtualvisa), but it's known to malfunction for long periods of time.
A 15-day visa is free for citizens from the EU, USA, Canada, and lusophone (Portuguese-speaking) countries, €20 otherwise. A 30-day visa costs €80. Given the problems that may arise in this process, it is best to apply early.
If you book a car rental or hotel booking through the travel agent Navetur, it will sort your visa for you – by far the easiest route. Otherwise you must deal with the closest embassy, of which there are only a few (some offer a mail-in service):
Africa Gabon, Angola, Equatorial Guinea, Nigeria
Europe Lisbon (covers Spain) and Belgium
USA New York
The turtle-shell jewellery sold in some souvenir shops is liable to be seized at the airport.