Many people have a standard set of items that they always take on a trip. But there are some items you need specifically for Iran.
Women should take a scarf to cover their heads. If you are a woman, all the time you’re outside in the public (so even in the common areas of a hotel) you need to have your hair covered and to wear a long-sleeved and loose fitting coat or tunic.
If you like real coffee every morning, you might want to take ground coffee with you along with a drip filter. Hot water is readily available, but not real coffee, even in the top hotels.
Extra toilet paper is a good idea, although it’s easy to buy in Iran. All the hotels have decent toilet paper, but not the public toilets you will encounter.
Make sure to take sun hats, although you can also buy them in Iran. Sunscreen is a good idea, especially if you travel during the hotter months.
A simple electrical converter is a very good thing to take. Some hotels have them, but better to take your own.
The Lonely Planet Guide to Iran is a good book to read before you go and also to bring along with you. We are recommending an earlier edition which appears to be more comprehensive than a later edition. Lonely Planet is an excellent source of information about places you will be seeing as well as the history of Iran.