Sen karanlikta aranan isik, kalbe atanan bir imzasin

not sure of aranan and atanan

ok I think I get it now .

You are the light that is to be searched for in the darkness

you are a signature that is to be written on a heart??

You are a light being searched for in the darkness,and a signature being assigned to the heart. 

I think-

aranan is the ´ing´ of the verb aranmak 

atanan is the ´ing´ of the verb atanmak 

My try

gerçekten  mi sagol

I´m sure if it´s wrong someone will pipe up!

I think it´s ´being.....´ not ´to be.......´

So it is

you are the light being searched for in the darkness?

I think so. Dilli corrected my translation-see above.