Student or Learner
Please explain, difference between "LET ME TELL YOU" & "I WILL TELL YOU".
Thak you for using correct punctuation on your post. In the future, you don't need to use all CAPITAL LETTERS in your posts. It feels like you are shouting at us.
In may cases, this may be the same thing. I think "Let me tell you..." may work in more contexts.
"Let me tell you..." could be seen as slightly more polite, because it suggests the person's permission to talk about this was given, instead of just assuming it's okay with "I will tell you..."
However, you tone can change that. "Let me tell you something, mister! You don't know me and you don't know anything about this. So shut up!" -- hardly polite.
Do you have a situation where you wanted to use this?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.