Publisher and literary agents are only interested in finished manuscripts - unless you're famous already.
So write and complete your book, then get a few friends and writers group members to critique it. Edit and polish it repeatedly. so it's as good as you can get it. Pay attention to professional formatting for MS.
Meanwhile, build up a web-presence which will become your potential audience of buyers. Research the publishing industry thoroughly. There are many relevant pages on this site, for example; and I've written a few too.
Approach a suitable agent, who will be able to help place your MS with a publisher.
Keep writing, keep persevering.