If you have something to sell, you have to be aggressive about your marketing and promotion. By "aggressive" I don't mean "launch full scale promotion attack on unsuspecting customers," I mean that you have to be proactive, have a plan, and follow through.
So you've decided to self-publish. Good for you! The first thing you need to know is that the astounding successes of Amanda Hocking and John Locke are not the norm. The second thing you need to know is that you are solely responsible for all of your marketing and promotion, so if your book doesn't sell, it's your own fault.
Even before you put out your first book, you need to get your name out there. People need to know who you are and what you're offering. By starting your self-promotion early, you'll also have time for trial and error. You'll be able to learn what works and what doesn't. You'll make connections--and in this business, just as in any other, connections can have a huge impact on your success rate. (It certainly doesn't hurt to make new friends, either.)
I suggest starting with a website, a blog, Twitter, and Facebook.
You need a base of operations. A home. Get a domain name and start your website. I like Intuit's website building software. When I worked as a bookkeeper and staff accountant, I used their bookkeeping software QuickBooks. It was so very user-friendly and they had great customer support, so when I decided to get a website--and knowing that I have zero graphic design skill--I went to them. When I signed up, they had a free trial offer. It looks to still be running. Their prices are reasonable, too. I signed up for a business website (which means I can have a store if I want one, which I do plan for) and privacy protection (because I used my parents' address in the States and I didn't want that information to show up on any WHOIS lookups) and it costs me $21 a month. This may not be reasonable for you; a regular personal website is $4.99 a month, and I believe you can upgrade at any time.
But for the sake of argument, let's say that you don't want to pay anything at all. Which is fine, because there are free alternatives!
Get a blog. Services like Blogger and WordPress even allow you to have separate pages in addition to your blog page, so you can set up a book page, a profile page, or anything you want. I think the main difference between the two is that WordPress is easier to customize, but I'm not sure, I don't have a WordPress. (It's on my to-do list.) You could also use LiveJournal or tumblr. Whichever service you choose, make sure you use it. Blogging is one great way to get your name out there and let people know what you're all about. Visitors and readers turn into customers, or they can turn other people into customers. Here on my blog, I try to offer my visitors things they'll find useful, so I typically blog about writing tips and self-publishing tips, and I point toward other blog posts I think are also useful. I also have a regular Monday Miscellany post, which I use for personal updates and non-writing discussion. Try to define what your blog is about and be consistent, both in content and in posting schedule. Also keep in mind that you're unlikely to be an overnight success. Just work to steadily build your readership.
Become a Twit. Twitter is one of the very best marketing tools on the web right now. It's a fantastic way to share information, interact with colleagues and customers, and show off your personality. Messages are limited to 140 characters, so you're forced to be creative and concise. I love it. But don't only use it for outright marketing and self-promotion. Let your followers get to know you. Interact with other authors, with book reviewers, with bloggers. Really use it.
As much as I hate to admit this, Facebook is important. You probably have a personal profile. I do, despite disliking the site (it's so unsafe) because we live so far from our family and friends and they all want pictures of the baby. I also have a public author page. Just type "create page" into the search bar at the top of the site and go from there. Use it. Update your status, share pictures, share links to your blog posts and to your website and to your book when it's for sale.
If this seems overwhelming, pick one thing at a time, get used to it and get good at it, and then incorporate another site. Eventually, you'll become efficient and proficient.