Not know, etc. the first thing about something/somebody
Definition 1 of 1
Pro Tip 1/3Figurative Use
This phrase is often used figuratively and doesn't require a literal first thing. 
I don't know the first thing about ballet.
Pro Tip 2/3Complete Ignorance
The phrase emphasizes a complete lack of knowledge, more than just basic unfamiliarity. 
He doesn't know the first thing about social media.
Pro Tip 3/3Expressing Humility
It can be used modestly to admit one's limitations in a particular subject. 
I'm a novice—don't know the first thing about chess.