You don't know me，You're about to. 

这是省略了已知的成分，等于：

You don't know me，You're about to know me. 

最简单的理解是，把 be about to 看作谓语动词的一部分。把 be about to 理解为构成将来时的助动词。类似 will do, be going to do, be to do 等。—— 这本来不是问题。不要随便找问题。

Never trouble trouble until trouble troubles you.