The question as posed is difficult to answer because it is not clear.
First, Iran has two leaders: There is the Ayatollah, or most senior cleric of Shiite Islam in Iran, and right now that is Ayatollah Ali Khameinei. He is a religious leader and holds the reigns to power in a more general sense. There is also the President who is "democratically" elected and right now that is Hassan Rouhani.
Second, it is unclear what "have to do" means. If it refers to the functions that they are required to perform for the office, they have to defend, organize, and determine the future of Iran. If it refers to what they should do in terms of policy decisions, there are as many answers as different people.
The spiritual and temporal Supreme Leader of Iran is the AYATOLLAH.
Ayatollah Ali Hoseini-Khamenei is the Supreme Leader for Iran.
The leader of Iran today is SYED ALI KHAMENAI
Khamenei
Iran has a president and vice president just like America.
Imam Khomeini
Ayatollah Alkhomeni
Saddam Hussein was the President of Iraq who invaded Iran during the Iran-Iraq War.
Reza Shah
An Ayatola is a religious leader in Iran.
Imam Khomeini
There was no leader before him Iran was a monarchy before the Islamic Republic and the king (natively called "Shah") was Mohammadreza Pahlavi.