There are two key ideas to consider before we get into the nuts and bolts of the "how" of social media.
First and foremost, the key word here is "social" and Rule #1 (and the only hard and fast rule if you want to be successful) is that the purpose of all of the different social media platforms is for you to build a community of like-minded people. People who, after they get to know you a bit and you show and interest in them, might be interested in the book you’ve written. Simply sharing links to your own books on social media—which some authors still commonly do—comes off as spammy and isn’t going to get you anywhere.
Secondly, social media isn’t for everyone. There is no rule that says you must have an active social media presence to sell your book. Reader’s feel when an author is being authentic vs forcing participation because they think they should.
So, if you aren’t truly enthusiastic about social media, there are many other ways to get your message out that might suit your temperament better: Speaking engagements or workshop for writers. Freelance articles or short stories. Writing blogs and offering content on your website (perhaps scenes that were deleted from your book—or a little side short story involving your characters. Recommending books on discovery sites like BookBub and Goodreads. All of that counts towards building buzz in your book, just as much as having some blue checkmark on Twitter does.
But if you enjoy social media (or if can hire someone to do it for you—always an option), here are some resources to get you started.
So You’re an Author Without a Social Media Presence: Now What?: This post tackles the questions: Do I have to use social media? If so, what social media sites should I use? What should I do on social media.
Social Media for Writers: Which Platforms You Need to be on, Based on What You Write: This post does a good job of analyzing social media sites by writing genre.
10 Tips for Authors on Using Social Media from a Literary Agent: This one stresses that the purpose of building a social media following is to build a community of like-minded people and, dare I say, fans. Social media will do nothing for you if you are only using it to try to sell books.