The Verdict: Clear Path
You are very likely a Croatian citizen already, and have been since birth. Your next step is to ask for your entry in the citizenship register and a domovnica.
Article 4 of the Croatian Citizenship Act makes a child born in Croatia to one Croatian-citizen parent a citizen by descent. Nothing is granted to you; it is established.
If you were never entered in the citizenship register, ask for a determination of Croatian citizenship (utvrđivanje). You file it at a police administration in Croatia or at a Croatian embassy or consulate. The form asks for your parents' details, where the family lived and when anyone left Croatia.
Once you are in the register you can get a domovnica (the certificate of citizenship), then an identity card and a passport. A child of yours born abroad must be registered before turning 21, so do not leave that late.