"I want to tell you" or "I want you to tell"?

" I want to tell you" means that I will speak and you will listen.

"I want you to tell (me)" means that you will speak and I will listen.

"I want to tell you" is a statement of intent. It means I have something to say, and I want to say/give it to you, I will say it and you will hear/read it.

"I want you to tell me " is a request, a desire, you know something I do not, and I want to know it. I will read/hear it, and you will speak it

For example:

You: "I want you to tell me the answer to my question"

Me: "I want to tell you the answer!"

Keep in mind, that they can be used as negatives e.g.

I want to tell you, but I can not