So, I happen to love this topic. My day job is working as the digital marketing director for a literary PR firm, so I get paid to help authors and publishers develop awesome brands online - primarily through social media! I've worked with authors across the board: from self-published first timers to indie presses to the big guys.
I've been working in digital marketing for over six years, with two years in the book industry (and I was the Director of Marketing for a mid-sized book publisher before my current position), so I've picked up quite a few tricks. Plus, I'm an author myself, so I spend a lot of time implementing these! :)
Here are my best tips:
1. Start small.
-- It's always better to start with one social media platform and grow from there than to pick up 5 and struggle to keep them afloat. Generally speaking? Your genre (and audience) defines which platform you should be starting with.
2. Follow the 80/20 rule.
-- This one is tough for some authors (and some marketers), but it's widely accepted online. Basically, this means that you should only post blatantly self-promotional content 20% of the time on social media (e.g. "My new book, #Title, is available now! Check it out on Amazon: [link]"). The other 80% of the time? You should be sharing content that is "topic adjacent" as I call it, meaning related to you and your brand as an author. Obviously, if you've written a nonfiction book about knitting, it probably doesn't make sense for you to rant about politics - because that confuses your followers. However, if you're a romance author, you can share cute articles about finding love, date ideas, funny stories, pictures, etc. It's not that you can't share about your personal life (you can), but be sure that the content you're sharing provides value of some kind.
3. Social media kinda sorta doesn't sell books.
-- Ooof. This is a tough one to digest, but most of the time you're not going to see a direct correlation with book sales from social media activity. Does this mean you shouldn't use it? Absolutely not! Social media is about making connections and building a brand. (AKA - selling yourself!) As an author, your focus shouldn't solely rest on "man, gotta sell this book right now", because you also need to be thinking about what comes next. Maybe you've only got 50 followers now, so only 3 of them will buy this book, but - as your brand grows - you'll have 200 followers when the next book comes out (and 20 buy it), then 1000 (and 200 buy it) and so on. You need to be okay with the growth mindset, because - as a first time author - that's what it takes. Even if you're published traditionally, you won't get as much support from the publisher as you'd think. In fact, many of our clients are people who say, "Well, my publisher didn't do X, Y, or Z, so now what do I do? They expect me to promote the book!"
4. Don't give up.
-- Social media marketing is a marathon, not a sprint. We tell our clients this constantly. Rome wasn't built in a day, and your book probably won't be a best-seller overnight. It's going to take time and effort. Stick with it and good luck!