Let me tell you something, my friend.
朋友,让我来告诉你...

这里这个let me tell you sth呢,是用以引入并强调将要说的话 used to introduce and empahsize what's to be said.

Let me tell you something, son. You're not going anywhere tonight.

我告诉你吧,儿子.你今晚哪儿也不能去.