I have been doing my family genealogy for several years now and have had a lot of luck with all of the branches except my paternal grandparents. My grandfather was Crist Nickoloff (possibly Hristo Nikolov). I know he was born in Sofiya, Bulgaria in 1890 and fled to "White Russia" at some point, from which he emigrated to the US in 1915. I know nothing else at all about him. I know about the same for my grandmother except that she was born in Poland and immigrated into the US in 1912. Without any other info, how do I go back any further? Where do I start? Thank you.