At the restaurant
As you might have guessed, you will not find alcohol in Iranian eateries. Instead you can try tasty local drinks, such as “doogh”, a yogurt-based beverage very popular in Iran. It is sometimes carbonated and seasoned with mint. Also don't miss Iranian non-alcoholic beer with all possible flavours.
Most restaurants, especially in the hotter regions in the south, serve lunch (12.00-16.00) and dinner (19.00-24.00) at certain hours and close for a “siesta”. In small cities knowing this can be crucial, because you will not find anywhere to eat during “resting” hours.
If you travel to Iran during Ramadan, the Muslim fast, you will see that all restaurants and cafes are closed during daylight. However, some of them work secretly to serve the travellers who can skip fasting.
It's a good gesture to tip your waiter, however, it's OK if you don't.