It is compulsory to have a travel insurance to visit Iran. If you already have one that covers worldwide, it is unlikely that it is valid in Iran, unless it explicit says it also covers Iran (you might want to call and check. In case they say they do cover Iran, ask for a written proof to show at immigration).
Otherwise, you can book a travel insurance to Iran at the airport, upon arrival. It costs around 14 euros, for 30 days. But, if you would like to have everything arranged beforehand and show the immigration officers you have done your research well, you can purchase your Iranian travel insurance for the same price or even cheaper through a local travel agency – and online .
