My publisher coached me on how to build my online platform. Get a website up. You can buy a domain if you want. I use a free Blogger site and it's serving me well. The main thing you want that website for is online visibility. Pick topics to blog about (and Tweet about) besides your book. Generally speaking, people are turned off by authors posting constant "buy my book!" messages. Talk about the writing process. Give updates on your progress towards your release. But also pick some personal topics to discuss. Comment on the writing industry. Post book recommendations. Blog about topics related to your book.
In my experience, I've gotten more followers and had more people buy my book because we've connected over something entirely separate to my writing.
One thing to keep in mind is that if you're self-publishing with a view to making a name for yourself and attracting a publisher, the odds are extremely slim that they will take on anything you've already published on your own, unless it's already selling very well. You would still likely have to send queries for new work you've completed that you don't intend to self-publish.
Whatever you do with your website, make sure it's published in your name. What I mean is, name your blog after yourself, or buy your own name as a domain, rather than coming up with a catchy website title or buying the domain of your book's title. You're creating a brand, and that brand is you. You want people to associate your name with your work.