Always with a negative particle: You don’t know jack shit. You can also say JACK without the SHIT: You don’t know jack!

Similarly, you can just use SHIT to mean “nothing”: You don’t know shit!

[woman]: What did you say to that mugger to make him run like that?!
[man]: I didn’t say jack shit. I just showed him my gun and he hauled ass.

https://voca.ro/1grSvGJnsScP