I think it’s best, to begin with a website. It can be simple to start if you don’t have any published books yet. This website is a combination blog and website using WordPress. I find it easy to use and manage. It’s easy building your website from a template to revising and maintaining it.
I am on Facebook, Twitter, and many writers are on Instagram and Pinterest
http://www.goodreads.com – You can be a member of this site for free and develop friends who are readers. Join groups and get to know people. Focus on readers who like to read what you write, as well as share with people who enjoy reading what you read. Develop an author’s page and post to the blog. Put your book trailers on your page. According to independent studies, more conversions to sales come from relationships on Goodreads than from Facebook and Twitter. Personally, I’m on all three, but working on spending more time developing my relationships on Goodreads. I am not only a writer, but a reader.
http://www.authorsden.com – Join with a basic membership for free and set up an author page. It includes your bio, photo, a listing of up to 4 books, stories, articles, news, your upcoming events, blog posts, and reviews. With upgraded memberships, there are additional features. You can also purchase advertising on the site.
These are just a few to get you started. It’s best to get started with one or two and do them well, rather than spread yourself too thin.