So You're an Author Without a Social Media Presence: Now What?
Jane Friedman talks about how to get started. She includes a number of links to other helpful articles on specific related topics.

14 Must-Do Social Media Tips for Authors
Laurence O'Bryan gives some quick and simple general tips, many of which are applicable to more than one type of social media.

Note that you certainly don't need to be active on every possible form of social media (and most authors don't have the time for that, anyway)! It's best to pick a few and focus your efforts on those. Here are some options to choose from, along with more articles you'll find useful when it comes to your online presence as an author. I think the titles speak for themselves. 

     a.) Website

How to Build the Ultimate Author Website (in 1 Hour)

Website Tips for Authors

Author Website Templates

     b.) Blog

Blogs: Elements and Design

140 Tools and Resources for Building Your Author Website and/or Blog

13 Blog Post Ideas for Novelists

10 Ways to Promote Your Book on Your Blog

How to Create a Social Media Marketing Plan for Your Blog

     c.) Facebook

How to Optimize Your Facebook Author Page to Sell Books

14 Ways to Promote your eBook on Facebook

     d.) Twitter

Twitter 101 for Authors: Hashtags, Dos and Don'ts

8 Twitter Resources for Authors

5 Things for Authors to Tweet About (That Aren't "Buy my Book!")

     e.) Pinterest

34 Strategic Ways You Can Use Pinterest to Market Your Book and Your Author Brand

From Fun to Serious: How I Use Pinterest Differently Now

     f.) Instagram

Simple Instagram Tips for Authors

Top 7 Ways Authors are Using Instagram

How to Use Instagram as an Author Plus 10 Ways to Grow Your Account Organically

     h.) Tumblr

Tumblr for Authors: How to Get Started

Tumblr 101 for Authors

Tumblr Tips for Writers

     i.) YouTube

Video Marketing and YouTube Tips for Authors

Beyond Book Trailers: 4 YouTube Strategies for Authors

     j.) LinkedIn

The REAL Power of LinkedIn for Authors

12 Ways to Leverage LinkedIn for Book Promotion

6 Top Tips on How to Make LinkedIn Work for Authors