Upvote:1
What determines the financial compensation eligibility under EC261/2004 is the arrival (time) at the final destination as specified on your ticket. Any such financial compensation does NOT release the airline from its obligations (to transport you to your final destination), i.e. the airline must still rebook you onto the next available flight and provide refreshments/accommodation if applicable. You should go to the airline's support desk at the airport to get rebooked/food/accommodation. For TK in Instanbul you could ask for a free access/entry into one of the TK lounges (which feature plenty of food and place to rest).
The best way to claim the financial compensation under EC261/2004 is to contact the airline directly. Each airline (operating flights from EU airports) should have a section on the website dedicated to claims under EC261/2004 (or at least provide details how to make such a claim).
Have a look at the wiki for a very decent summary of your rights