Do I have to declare overseas real estate?

I heard from friends that overseas real estate should be declared on taxes, but I do not understand why it has to be done or how to go about declaring it. 

 

I am a green card holder living in US. I am also a citizen of Kazakhstan and own two flats in that country.  I do NOT get any income from those flats. I owned those flats for years, before even coming to the US.

If I need to declare ownership of those flats, who determines their value and based on what?  How would I declare ownership in TurboTax? Thank you!