Set up a mailing list and work on growing it, so you can send an announcement to everybody when your book goes live (some people won't buy on preorder) and so you can announce when the sequel (write one!) is released. All your efforts are best directed at writing the next book and trying to get reviews for this one, so you can start advertising it once you have the sequel(s) out. But if you want to do additional promo, I suggest choosing something you enjoy, like Facebook, setting up a page, and building a following there. With everything you do, try and drive signups to your mailing list and try to get people to review. That's laying the groundwork for the future, when you'll have more titles to work with and more options.