"No sé por dónde empezar a buscar."
Translation:I do not know where to begin searching.
For those of you who can read in spanish here is some information about ¿Por dónde..? / ¿Dónde..?:
As I could understand with my spanish, it seems that 'por dónde' is used when talking about DIRECTION, ROUTE and 'dónde' when talking about the POINT where you are aiming your path.
So when I say "I don't know where to begin searching" technically it means 'I don't know in which DIRECTION to keep my search'. Who agrees?
Try to read the link yourself and tell if you figured it out the same way.
We not longer use the word "whence" = "from where", but in this sentence it would be a more accurate translation for "por dónde": "I do not know whence to begin searching".
Note: English also has "hence" = "from here", which is not completely outmoded as it is used in the phrase "hence forward (in time}", e.g., "hence forward we shall hunt only with a camera".
"Hence" is also used as a synonym for "thus", e.g., "I am tired, hence I shall sleep".