I use a publisher, because I am exceedingly untalented as an artist, and because I know I need an editor, and a proofreader, and all that jazz. But my first publisher was all about self-marketing.
I like Twitter, because you can schedule a tweet and have it released weekly, or daily if you’re at release time. Facebook is great for launch parties, and I did a Rafflecopter giveaway thing, and all sorts of fun stuff. If you have a blog, blog about it and link the blog to other places, like an Amazon author page (free) or a Goodreads page (now owned by Amazon, but they pretend to be separate).
As far as where to sell it, even if for free, I would strongly advise not to have anything to do with Kindle Unlimited. Keep your options open, so you can distribute your work on other sites, like Smashwords, Barnes & Noble, or via your blog itself, if you’re so inclined. KU will restrict you to only their service, and when you decide to offer something for money, they have a very odd algorithm to determine how much you earn, one which is not fair to the authors in my opinion. Kindle itself is fine, but Kindle Unlimited is not.