At www.werk.nl you find extensive information on working in the Netherlands in French, German, English and Spanish. At JIP they also have a brochure about working and studying in Rotterdam.
To start working in the Netherlands, even for a short time, you will need to obtain a Citizen Service Number (NL: Burgerservicenummer, in short BSN). With this personal registration number you are registered with the tax and social security systems and you can pay taxes. EU/EEA citizens need a valid passport/identity card in order to obtain a BSN. Non EU/EEA citizens need to show their work permit and contract from an employer.
If you register at the city hall, you will automatically get this magical number. If you don't have an address and can't register, you need to make an appointment with the tax office to get it. You dial 0800-0543. The Dutch speaking computer will tell you to press 4>3 and after that a real employee will start talking to you and you can make your appointment.