How to buy Bitcoin in Iran
You will need to find an exchange that accepts your bank account or method of transferring from your account to the exchange.

A number of exchanges out there not personally sure which would accept your bank, but another option is Local Bitcoins

Here is the link for Iran https://localbitcoins.com/country/IR

UPDATE 1st June 2019: Local bitcoin is now banned due to new sanction.

Iranian can't use localbitcoin in Iran anymore but they can buy and sell here as an alternative: 
https://www.keepchange.io/buy/bitcoin/IRR?