I agree with the previous advice on nonfiction--You need a proposal and sample chapter. What the agent/editor will look at closely is the marketing section. What else is out there, how did it sell and how do you realistically plan to market this book?
For fiction, you have to write the book first. An agent will get you far more attention with editors with nonfiction or fiction. Most publishers refuse to look at nonagented stuff unless it's straight genre (such as category romance).
Self-publishing...sigh. I don't think the self-published have paid their full dues and there's LOTS of self-published crap out there. Anybody can get a book printed, few can get a book printed by Penguin, Simon&Schuster, Warner or Randomhouse. Those are the credits that count. Most agents and publishers don't count self-publishing as being really published. Money should flow one way, towards you. After getting 24 books published, I haven't spent one penny.
I'm traditionally published with an agent and that's what I recommend. Try to get published-for-real first. Do your research! If you're loathe to do your research, forget on publishing in nonfiction. The proposal itself will take exhaustive marketing research.
The question you asked is answered in spades in any library and on the Internet. Don't ask authors elementary questions, to read your stuff or for them to intro to their agent. If we don't know you it opens us up to legal hassles.