The easiest way to travel to Iran as a tourist is to go through a tour company. Look up "Touring Iran" or "Traveling to Iran" and several tour company websites will appear.
The tour companies will often ask you for your information, including passport information, passport photos, your reason for travel and even a CV. You will pay the tour company a processing fee and they will send your visa application to the Iranian government.
After the tour company submits your application to the Foreign Ministry in Tehran, the Ministry will send the company an authorization letter with an authorization code. The company will pass the letter on to you.
With the authorization code, you should submit your passport to the Iranian Interests Section of the Pakistani Embassy in Washington, D.C. You will need to submit two passport photos. If you are female, your hair MUST be covered in the photos. You will also need to submit a visa fee. Fees vary based on nationality; as of June 3, 2008, fees for Americans are $104. If you are sending your passport by post, you will need to include $20 for a return postage fee. You will also need to include the visa application form No. 101, which can be found here: http://www.daftar.org/forms/visas/101.pdf If you are under the age of 18, you'll need to submit visa form 102, which can be found on the embassy site, as listed in the "Other Resources" section below.http://www.daftar.org/forms/visas/102.pdf
You will need to choose a pre-designed tour from the tour company or design your own itinerary in conjunction with the company. Especially if you are an American, you will in almost all cases be required to stay with your guide on the tour for purposes of security. The tour company will usually plan your meals, hotels and book your plane flights.
Traveling to Iran independently to visit friends or relatives can be much trickier than traveling with a tour company. You will need to ask the person you are visiting to submit a request for your visa to the Foreign Ministry in Tehran.
Your friends or relatives in Iran will then receive an authorization letter if their request for your visa is approved. They will then send the letter on to you.
Repeat Section 1, Step 4.
You will need to buy your own plane ticket. Even if you are granted a visa to make an independent visit to Iran, there are no guarantees that you will be allowed to enter the country once you arrive.