That is a strict rule of entering Ukraine to have student visa for international student in order to start studying in Ukrainian university.

With regards to general regulations of international education provided by the Ministry of Education and Science of Ukraine, Ministry of Foreign Affairs of Ukraine, State Boarder Guard Service of Ukraine, State Migration Service of Ukraine the procedure of admission for international student shall be as follows:
– an international applicant must receive the INVITATION FOR STUDY (strict security form, registered with name and personal data of an applicant, signed and stamped by the University)
– an international applicant must apply to the Embassy of Ukraine in his native country or in neighboring country for a student visa in order to enter Ukraine as a student

In such manner an applicant must prepare the full package of documents requested by the Embassy with the INVITATION FOR STUDY first and foremost.
In general it is called to provide originals and legalized translations of birth certificate, education certificate, AIDS and general medical certificate together with photo, insurance policy and filled application form. Still we strictly recommend you to contact directly the embassy before starting application process, following the given link: Ministry of Foreign Affairs of Ukraine