Literally, this says "I say you what"? Generally, "我说" is simply an introduction to a quote you said or just a filler introduction (it's like saying "I say" at the beginning of each English sentence; it's not needed, but some people simply speak that way). "你怎么“ can't really stand by itself. It needs something else to have some meaning. For instance: ”你怎么了？“ is an expression used to show concern for the other person (the equivalent of "What's the matter?" or "Are you okay?" in English). "你怎么样？” means "How are you?" I hope this helps answer your question. If not, please let me know.

Why have you done sth? This phrase is used when you wonder why sb else has done some stupid things. it's kind of blame, which means you strongly disagree what he/she has done.

why do you have to .......