There is two easy way to get Iran tourist visa and In general, the maximum validity of traveler’s stay with tourist visa is 30 days and they are regarded as single entry.
Directly from Iran E-visa website
Travellers from more than 50 countries who wish to visit Iran can apply for an Iran visa through Iran E-Visa website. It’s the cheapest way to get visa. The Application is submitted electronically to Evisa Department, Ministry of Foreign Affairs website via the Internet. You should wait until your application pass all required process within 10 business days. It’s the cheapest way to apply for visa. In this process you just pay for visa stamp at the airport or Iran Embassy on your country. The fee is different based on the country you are coming from.
You can apply for Iran tourist visa through travel agencies. There are some Tour, Travel and Visa agencies in Iran with an official authority from Iranian foreign ministry to process and obtain travel permits to Iran. Applying for Iran tourist visa through travel agencies is a faster way to get visa but it’s not cheaper than applying directly from E-visa. People who are not going to get troubles almost prefer to apply through travel agencies. You should pay about 15-20 Euro as service fee to travel agencies and process of getting visa usually takes 2 to 10 business days.