How can I book flights online with separate airlines? [closed]

You have to make two individual bookings, you can't book this on a single ticket.

You can make each booking at any of the typical websites: Lufthansa.com ryanair.com, kayak.com, expedia.com, orbitz.com, hipmunk.com, google flights etc.

Please note that you are responsible for any missed connections: if your flight from Teheran is late and you miss the flight with Ryan Air, you may have to buy a new ticket or get stuck in Frankfurt. Make sure you have plenty of time.

Please note also that Ryan Air often uses Frankfurt Hahn Airport. That's a marketing ploy: Frankfurt Hahn is about two hours away from Frankfurt and closer to Luxemburg than to Frankfurt. Make sure you book Frankfurt International Airport (FRA) and not Frankfurt Hahn (HHN).

There is a difference between true "connecting flights", which are arranged as one booking, and buying several different tickets and pretending that they are connected flights.

The exact itinerary you gave is not bookable as connecting flights. (See Hilmar's answer for details.) This is because RyanAir does not cooperate with any other carriers. You have to buy the tickets separately.

For true connecting flights, even between different carriers, it is straightforward to book either through one of the airlines directly or through a travel agent. I had a look and you can buy a seven day return ticket from IKA to IBZ for 500 USD leaving next week on British Airways and Iberia as a single ticket. This can be booked on Expedia.com or any major internet travel site.