If you are a national of an European Economic Area country* or from Monaco or Switzerland you may travel to Belgium on the strength of a national passport or identity card.
If you have any other nationality, you have to apply for a visa (type short stay) with the Belgian embassy or consulate in your current country of residence before leaving for Belgium.
*EEA: EU + Norway, Iceland and Liechtenstein
Upon acceptance to a summer school, you can request a visa invitation letter through Mobility Online.
More information on the application procedure and visa requirements can be found here and here.
Addresses of the Belgian embassy or consulate in your home country can be found here.
For some countries, approval by the Belgian authorities can take up to three months. Make sure you start the visa application procedure well in advance!