Okay, you read the other blog that said MONTHS BEFORE you publish your book you need to get your foot in the door of social media. Right? You read that, didn't you? If not, it's up there ^ one post above. Read it. Obey me.
Alrighty then (no, that's not a word. I'm using creative license here. Give me a break, all you OCD freaks). I would suggest starting with Twitter. It's free and easy to open an account. Once you're up and going, do a search (you'll see the "search box" easily enough) for whatever genre you write in. For me, I searched "romance authors". That should give you a jumping off point. Scroll through the list and look for non-people types, those that seem to be more like a company. Or, you can cheat and go to my list of author-helpers here. There are quite a few people in that list who live just to help us lowly authors. They're awesome, amazing Tweeps (Twitter speak for Twitter people). Just click on a name and then click on "follow." Do that for as many people as you want and you will soon get "follow-backs." It's a very quick way to get friends on Twitter.
Now, once you're established on Twitter (give it a few days), move on to Facebook. Search for your Twitter friends on Facebook and send friend requests out. Do searches for authors like you did on Twitter. Look at other peoples' friends lists. Send out friend requests to them. Facebook is slower to get established with, but it will come. Once you have a hundred or so friends, make an author page on Facebook. (If you don't know how to do this, go through Facebook's "help" section and search "Create a Page".) Once you have an author page, invite your Facebook friends to "like" it, and announce it on Twitter.
Goodreads. Easiest one ever. Just add friends via your Twitter and Facebook accounts! Once there, join groups. It's easy to do and there are plenty available so you're sure to find one in your genre.
Then there's Pinterest. I have a personal Pinterest account, but I haven't actually started one for my business (author) side *embarrassed coughing*. Be right back. SWOOSH (sound effect of a superhero cape). Same theories apply. Open the account, start pinning (posting) stuff. Mention your book. Don't overdo it.
There are many other social media sites (LinkedIn, Google+, StumbleUpon, etc.) but once you're established with those I mentioned, getting going with the others is a piece of cake--preferably devils food with homemade fudge frosting. And a big cup of extra-dark Rwandan coffee. Ummm, yeah, baby.