Since Rotterdam is part of one of Europes largest agglomerations, the Randstad, it is very well connected to the important places of western Europe. Try some of the following:

 

Budget:

Eurolines. This pan-European bus travels from almost everywhere to everywhere. It's like the Greyhound of Europe (if that rings any bells). Very, very cheap but unpleasant. The trip takes very long, because you'll have to pick up people in all kind of places. But cheap.

 

Transavia. This Dutch low-cost airline flies to Rotterdam Aiport from selected airports in Europe and beyond. If possible, this is the best way to travel to Rotterdam!

 

Mid-range:

NS - National Railroads. Train travel isn't cheap. But it is much more comfortable than a planeride or cartravel. Try it.

 

Amsterdam Schiphol Airport. Lots of low cost airlines fly from different locations to Amsterdam Airport. A 50 minute trainride (ca. 12 euro) brings you from the airport to Rotterdam.

Low-cost operators at Schiphol:

Easyjet, Ryan air, Transavia, ah, you can find them yourselve at www.lowcostairlines.nl.