As Bernhard managed to find, there's information on public transport on the airport's website (click the British flag in the top bar if it comes up in Dutch). It's actually even easier than the website makes out!
In the baggage hall, there's a sign up explaining about the two bus options, and with directions to the bus stops. When you exit the terminal, turn left and walk for an epic 20-30 meters, and you'll reach the bus stops. There's one stop for the #50, one for the #33, both well signed. If you have an OV-chipcard, you can use that for the journey. Otherwise, €3 in cash paid onboard gets your a single ticket valid for 1 hour.
The #33 bus runs direct to Rotterdam Central Station, taking about 25-30 minutes. For the west of the city centre or around the station, that's probably the best bet. Otherwise, the #50 bus shuttles you out to the Meijersplein metro station, from which it's easy to take the metro north to The Hague (Den Haag), or south into the city centre. You can use a journey planner like the Rome2Rio one or Ret.nl to work out which'll be the best for you. Both buses are pretty frequent, every 10-20 minutes depending on the time of day, and you can be at the bus stop in well under 10 minutes from wheels down if you're travelling only with hand luggage only
Depending on how many journeys you plan to take, it might or might not be worth getting an OV-chipkaart. It's currently €7.50 for one, which reduces the single ticket to the city centre to €1.66, so you'll need to make a few journeys to make it worthwhile. If it's open, you can buy one from the Ako newsagent, which is between the arrivals and departure areas (turn right outside customs, and it's on your left next to the currency exchange office). However, Ako seems to have much shorter opening hours than many other places in the airport, so take care if arriving very early or late in the day! Otherwise, the people at the information desk just after you exit customs are very helpful, ask them for advice if you need it!
