I am shocked how some are completely oblivious about other religions. Like not knowing what a Mikveh is.
You can start with things that one might assume are more common knowledge, but most people are oblivious to, such as Shabbos observance, Kashrus, Torah education, etc. etc.
Knowing about something that is outside one's realm, will come from either getting a targeted education about it, or from encountering something and following curiosity. Being that many of us live in Ghettos, many have never seen certain things until a later stage in life (and sometimes not even then).