Take a metro from Rotterdam to Spijkenisse Centrum. They are very frequent, and it takes about 30 minutes.
From there take a bus 101 to Hellevoetsluis.
All in all your travel time is 1 hour.
All public transportation schedules can be found here:
NB: I would be able to be more specific if you let me know where exactly you need to be in Hellevoetsluis. With the above, I have assumed Rotterdam Central Station to Hellevoetsluis main shopping center Struytse Hoeck.
If you need more specific information, please ask.
Here's a Metro map Rotterdam region
The Netherlands has a vast and efficient network of freeways known as the "Snelweg". Destinations are very well marked with plenty of exit warnings making it easy to get around. The speed limit on the Snelweg is 120 kmph and don't forget to drive on the right.
All parking is free in Hellevoetsluis. You will have to mind the parking spaces and sometimes there's a maximum time (though only behind the town hall, if I'm informed correctly).