First of all, there is no guarantee that you will get rich doing this, but there is a guarantee that you will have to work a little harder (or longer) to make it work. You will have to learn to do graphics to create your own cover, or else pay someone else to do it. You will have to learn how to create an e-book file or pay someone else to do it. You will have to find a beta group of readers or a professional editor to help edit your book, and that may cost money, depending on which you use.
As an example, I have two fantasy novels I have self-published as e-books. I paid $100 for each of the book covers, and that was it. As an IT professional, I had no problems figuring out how to create the e-book files. I have a family member who is a professional editor, so she did that for free. I also have a group of four beta readers who helped critique and edit the book as well. So my total cost for each book was $100, and I made that back on each book in a month or less. I now make a profit on every book I sell. It isn't a large amount, but it is increasing every month by about 5-10%, and I do absolutely no promotion.
My example is not at all uncommon. In fact, I can point you to at least 200 people who are actually doing BETTER than me, and virtually all of them started with little or no experience. (I belong to a writer's group with over 300 active members, all of whom have become self-published authors within the last six months.) You can do a Google search on names like JA Konrath, John Locke, Amanda Hocking, and Blake Crouch to find examples of people who are selling hundreds of thousands or even a million e-books! My examples are much more modest and realistic for the "average" unknown writer.
Everything you need to know to do this yourself is available online, and I would be more than happy to help point anyone in the right direction to get them started. (For starters, try "Let's Get Digital" by David Gaughran.) It isn't difficult, and it does NOT cost a single penny to publish. If you choose to go with print on demand (POD) service like CreateSpace, you can still publish at no cost. (You may have to purchase the first proof copy, but you'll probably wnat your own copy anyway!)
To get back to the specific question, your self-publishing options that I highly recommend and that I see most authors using are Kindle Direct Publishing and Smashwords. KDP is Amazon's e-book publishing service, while Smashwords is a distribution company that will create half a dozen different types of e-books for you and then distribute them to Barnes & Noble, the Apple iBooks Store, and Sony readers. For print services, I highly recommend CreateSpace, because it is free and easy to do.
Pros: You have full control of your e-book. You decide the cover, you decide the price, you make all the decisions. Also, once you publish, it stays out there forever. With print books, they will stay in a book store for 1-6 months (assuming you get picked up and distributed by a publisher), and the ones that don't sell get remaindered and then never seen again. Also, the royalties for e-books is 70%, compared to 6-17% for traditionally published print books. E-books currently outsell ALL paperbacks as well as hardbacks
Cons: Since you have full control, you also have full responsibility. You have to obtain or create the cover. You have to pay an editor or find one for free or take your chances without one. You have to do all the marketing and promotion. (However, traditionally published authors will tell you they do a LOT of this themselves anyway.)
BTW - The best form of promotion is your next book. If people like the first one, they are going to be looking to see if you have anymore books out there. If not, you're missing a sale. My sales for my first book increased after I published my second.
Reasonable expectations: Depends on the type of book you have written and the amount of social media buzz that gets genrated around it. For a mediocre book you can probably expect to sell a dozen or less in the first month. If you do any promotion, you might see that double the second month. From there you might get 5-10% increases each month.
If you write a great book, then word of mouth will help you sell more than you could ever imagine. Then you could be looking at hundreds or even thousands a month. (Don't scoff, I can name at least a hundred "average" writers already doing this.) The key is to write a good book, make sure it has a good cover, write a catchy blurb that draws people in, and then hope and pray it catches on.