All major mobile phone companies have so-called "start packages" for prepaid users. You can buy them practically everywhere - in kiosks, post offices, shops that display "partner signs" of mobile companies.
Most start packages have the price of 100 Kuna, which you can spend on your calls after you activate your account.
Here are some links, T-mobile and VIP have websites in English:
T-mobile: Simpa start package
Vip: VIPme start package
Tele2: Bla bla start package
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