The Rotterdam Transportation Authority (Dutch: Rotterdamse Elektrische Tramwegmaatschappij, RET) provides mass transit services in the greater Rotterdam area by tram, bus and metro. To travel, you need a public transport chipcard (Dutch: Openbaar Vervoer chipkaart, OV-chipkaart). Personalised, anonymous, and/or disposable cards can be purchased online, from RET ticket offices and from RET ticket machines at metro stations. A card costs €7.50 and you always need credit on it to be able to use the card.
At the RET-website you can find a map with an overview of all the metro lines.
If you would like to plan your journey, you could do so using the 9292OV-website.