Most of the visitors to Iran must obtain a visa from one of the Iranian diplomatic office around the world unless they come from one of the visa exempt countries or countries eligible for a visa on arrival apply for all Iranian International Airports by E-VISA System.
See this link http://e_visa.mfa.ir/en/visa-arrival/
Citizens of the following 11 countries can visit Iran without a visa.
All visitors must hold a passport valid for at least 6 months.
Admission is refused to women not wearing Islamic head cover, scarf, long sleeves and stockings.
Since 2017, Iranian authorities are exploring a potential visa waiver program under which visa limitations for 60 countries will be lifted as part of Iran’s program to attract more tourists.