Formalities

You need a visa, a real one  that you need to arrange beforehand or have before you arrive.
Israelis are not allowed entry in Iran. A handful of countries do not need a visa. The electronic visa (VOA, visa on arrival) is available to all others except the USA, Canada and the UK mainly. Citizens of these countries need to get a real stamp visa on their passports BEFORE they arrive.
Other travellers that need a visa can have it prepared electronically and claim it as they arrive, in a rather surprisingly straightforward procedure. Your agency, if there is one, will most probably prepare a batch for their traveller.
Israeli entry stamps are prohibitive. Suspect stamps of entry and exit points near Israel as well. You will have to wait until you get a new passport.
On the other hand, an Iranian visa will ban you for life from the ESTA visa procedure for simplified entry in the USA. Do not even think of lying even if you have a new passport. Despite the fact that the electronic visa consists as of 2018 in a simple email (nothing stamped or attached to your passport), lying is a felony while entering in the USA and the servers that hold almost all passenger lists (PNRs) are on American soil.
If you have already been in Iran and have to travel to the USA, you need to first fill in the ESTA form and then ask for an interview at an American embassy. You will need to answer very penetrating questions (as i.e. are you intending to prostitute yourself while in the USA?...) but usually you will eventually get your visa. Which of course is not a guarantee that when you arrive there you will be allowed entry…

Other stuff

European electricity plugs. Water potable but best avoid. Bottled water really cheap, but think of plastic pollution as well…
No alcohol whatsoever in Iran. In the best case you will get a beer without alcohol full of sugar for your meal. While in Iran you may realise how much we associate certain pleasures with alcohol.
Espresso seems to be more or less possible nowadays. Café Shazdeh  is one of the chains that use yellow antique VW vans as selling points.
Tipping is not very common, except in bigger cities.
Do not in any circumstances use the thumbs up gesture. It means totally different in Iran…
Although this may be a personal opinion, better not engage in religious conversations, if they occur. The gap is simply huge.
Facebook does not work. Your only social media possibility is WhatsApp, used by everybody in Iran. And of course monitored closely, but that seems to be an international phaenomenon more or less.
Bring appropriate clothing, especially if you are a woman. Choose your scarf, which you will use from the moment you leave the aircraft. Skirts are not allowed. Men can dress almost in the same way as in the western world. But shorts are not allowed.
Wifi is as a general rule good to very good.
Photographing is relaxed, although of course some people will hide their faces. Photographing official buildings or military installations or their vicinity can be problematic. Your guide should tell you when to stop using your camera. Iranians absolutely adore selfies. Each moment outside is precious and has to be kept eternally through a pic. Use this opportunity to speak and make friends!