The Netherlands is a member of the Schengen agreement. Depending on your nationality and the length of your stay you may need a visa to enter the Netherlands. Please see the website of the Dutch Ministry of Foreign Affairs for more information: https://www.netherlandsandyou.nl/travel-and-residence/visas-for-the-netherlands .
You can apply for a Schengen visa at the Dutch embassy or consulate in the country where you reside or are entitled to reside. In some countries, you may apply for a visa using an outside agency. You can find the addresses and websites of the Dutch Embassies and Consulates here: http://www.government.nl/issues/embassies-consulates-and-other-representations
It is your own responsibility to apply for visa, Utrecht Summer School doesn't offer any assistance other than a Letter of Acceptance. We can only provide Letters of Acceptance after the course fee has been paid completely and has been registered.