Hmm...there's a lot, and I'm still learning. But a small sample would include this:
First, I don't believe it's a good idea to "choose" a publisher. Most people who publish for self-publishers are vanity presses. They charge you a lot for what you can do yourself. If you can format a Word document, you can publish on Amazon and Smashwords yourself.
As far as formatting goes, even if you're not going to use Smashwords, I recommend going to their site and picking up their free ebook for self-publishers. It has a lot of technical know-how on the correct way to format your Word document for publishing.
The real trick, other than writing a good book (some of that's in the eye of the beholder, but at least don't write an embarrassingly bad one), is coming to grips with promotion and book reviews. What's your plan for succeeding in this area? If you don't have a plan, your book will probably sink like a rock, no matter how good it is. Okay, you might get lucky, but luck is not a business plan.
And if you're a self-published author, like it or not, you're in business for yourself. You'll have to come to grips with all the associated tax stuff that goes with that.
Promotion is particularly difficult because everyone seems to do it a bit differently. There are a lot of options, and it seems like everyone dials in their own mix. As a for-instance, I've found Pinterest to be particularly valuable. But I write fantasy, and fantasy fans traditionally have a keen interest in images associated with the genre--pics of dragons, unicorns, etc. If you were a thriller author, you wouldn't likely find Pinterest as useful as I do.
In a separate but related area, how will you get reviews? Books are easier to promote when you have them, and Amazon has recently changed their standards in this area again. Opinions will differ here, but I'd go for a soft launch--having the book technically for "sale" as a free ebook until you get enough reviews to make promotion worthwhile. But just being free won't get the job done by itself. Again, you need a plan here. I gave away three thousand copies of my book and netted seven spontaneous reviews out of it. I had to get proactive--contacting reviewers one by one. And these reviewers need to be targeted correctly. There's really no point in sending out my book to someone who only reads paranormal romance.
Other people will chime in with more, but figuring out how to get reviews and effectively promote seems to be the real trick. The book promotion subforum here is a good place to start. I've blogged a lot on this particular topic, as have many others here. Start wading through articles and try stuff you trust.
Be careful about spending money. I won't say not to spend any, but there are a lot of online "classes" out there--mostly just ebooks and videos--that'll charge you hundreds of dollars for this stuff. And a lot of it's already out there for free online, on blogs and youtube videos.
Hope this gives you some ideas of where to start.