The first piece of advice I would give to aspiring writers is to be realistic about your writing goals. Are you writing to share with your family and friends or with the whole world? While it should not determine the work you put into the manuscript, it will definitely impact what you do when the manuscript is complete.
The second piece of advice I have comes from the publisher of "The Shack", who once said the distinction between traditional versus self-publishing should be erased in favor of the distinction between professional and unprofessional publishing. No matter whether a major house is publishing your book or you are availing yourself of the wonders of self-publishing, your book should be well-researched and written, thoroughly edited and critiqued, have a solid cover design and be well-marketed and distributed. You must realize you will do a lot of this work yourself, even if you are picked up by a major house. What you put into your work is what others will get out of it.