A valid passport is required. Visas are not necessary for citizens of EU countries, the U.S.A., Canada and many other countries. Visa requirements are given here.
As a participant with paid registration an official letter-of-confirmation that confirms that you will attend the BtS2017 will be sent to those that need it for obtaining Croatian visa. This letter does not guarantee that you will receive a visa. It is recommended that you start this process as soon as possible!