The phrase "not know the first thing about (someone or something)" means that a person has no knowledge or information about a particular topic, activity, or person.
Here are a few
examples:
1.
She didn't know the first thing about playing the piano,
but she decided to take lessons and learn."
2. I don't know the first thing about gardening. I am allergic to plants.
3.
She doesn't know
the first thing about football, but she still enjoys watching the games with
us.
https://dictionary.cambridge.org/zht/%E8%A9%9E%E5%85%B8/%E8%8B%B1%E8%AA%9E-%E6%BC%A2%E8%AA%9E-%E7%B9%81%E9%AB%94/not-know-the-first-thing-about-sth