There are advantages to both, but if you believe that promotion/marketing is an advantage to traditional publishing, think again. Even the best known writers have their own Facebook pages and/or blogs to keep in touch with their fans - and to market.
Traditional - Polish your manuscript (self-editing) using The Elements of Style.
Write a query letter (see Query shark). The letter should be one page, detail the type of book and length, contain a short description, and an introduction to the writer. No cheating by changing font size to fit - should be TNR 11 or 12 or a variation thereof. Then wait. Polish book one or write book two. Some publishers take up to 90 days to respond. (One took over a year to send a rejection notice. By then I'd already sold the book to someone else.) Wait for rejection, request for partial or full. Send EXACTLY what they request. Wait for rejection, acceptance, or request for revise and resubmit.
If accepted, prepare for the editorial process. Most editors are on your side, their job is to make your manuscript shine - and that's not just spelling and grammar but repetition or unnecessary detail. That being said, be ready to kill your darlings. If that perfect sentence doesn't advance the story, it will be cut. And you may be asked to make major changes. Remember, the editor works for the publisher, not you.
Once the editing process is complete, a release date will be announced and a cover created. (You may have some input into the cover - don't expect a cover artist to recreate a scene in your head on the cover - but the publisher has the final say.)
Market, market, market. The publisher will expect you to create and maintain a web page, Facebook author page and Twitter account. Use them. Announce release dates. Arrange promotion via FB sites. When release date arrives, promote but don't spam. The best marketing for book one is book two, etc.
Self Publishing - Technically, you can create a document in Word and a cover yourself, and put it up on Amazon or Smashwords. Don't do this. A poorly prepared book with an unprofessional cover is an announcement that you're an amateur.
If you haven't already, at least write book two.
Meanwhile, for book one. Find beta readers, an editor, formatter and cover artist. Polish your manuscript as above. Beta readers are readers willing to look at your manuscript for free to catch basic errors in grammar and continuity. Send to the editor of your choice. Here is where Self publishing has the advantage - the editor works for you. However, the job remains the same, to make your manuscript shine. Ignore their advice at your peril. Choose a cover artist - look at the samples on their web page, find examples you like from books in Amazon or Smashwords. Don't expect a cover artist to recreate a scene in your head for the cover - they aren't mindreaders. Most will ask for the blurb (short description) of the book, and details of main characters. Have no expectations. Consider using premade covers for the short term. Send edited manuscript and cover to formatter (or learn to DIY).Create and maintain a web page, Facebook author page and Twitter account. Use them. Announce release dates. Arrange promotion via FB sites. When release date arrives, promote but don't spam. The best marketing for book one is book two, etc.