Google only gets you so far. And Wikipedia is only if the person is popular or famous. Ancestry.com is your best bet IF the person has been dead for a long time. If not, try Classmates or similar sites.
Since (at least here in Denmark) the child's name along with the parents' name is written in it. If you know the name of your greatgrandfather, but lack more information, finding the right churchbook will help you on from there.
Since (at least here in Denmark) the child's name along with the parents' name is written in it...[/quote]
That's the same everywhere in the world, I think. Most people seek information about their ancestors in the church books. You can't expect to find everything online.