Dear Konuk,


I guess you are also a Turkish citizen.
1- Of course, you will be in need of a proof of heir which can be obtained from a civil regitration offices. Then, you will be in need of your father's death certificate. Toethr with those dcuments you should apply to Turkish courts to get certificate of inheritence.That certificate of inheritance will show your ownership on your father's heritage (if your mom is alive or if you have siblings they will be writen as allottees)
2- In order to get heritage of your grandpa, you will also need certificate of inheritance issued on behalf of your grandpa and showing your father's percentage on his heritage.
3- Together with those, you can go to banks and land registry offices to see what he owns. P.S. As a condition precedent to obtain that heritage, due taxes should have been paid to the tax offices.
4- "Power of Attorney" which corresponds to the turkish word of "vekaletname" is necessary to do all of these activities. However, providing such a power of attorney to your relatives is always dangerous considering the fact that they may lie to you regarding the things that yourgranpa owns.
Therefore, I suggest you to engage with a lawyer based in Turkey yourself and have those things done by your own lawyer in order to prevent any loss of right. On the other hand, if you trust your relatives that much it is of course your discretion to provide them with that power of attorney.
Best Regards