Are both sentences OK?
Never take anything from anyone you don't know.
Never take something from someone you don't know.

Question

Answer

Both are okay, but I would want to say: "Never take anything from someone you don't know."

That way you're not using "any" twice in the same sentence. To me it sounds better :)

Answer

I would be tempted to say:

Never take anything from anyone *that* you don't know.
Never take something from someone *that* you don't know.

Both sentences are ok however. In particular, the first sentence has more emphasis:

"Never take //anything// from //anyone// that you don't know."