Social Media really is key. Having a great website where readers can connect with you also helps with your professional appearance as well. But the great news is, there's plenty of different platforms for you to try a variety of ways to connect. Facebook is simple. Short text posts about your life, writing etc. Maybe a cute photo of your dog, your hike, whatever. Just something to let readers know what you're up to. Personally, I post short writing snippets of my work on there to get people interested. It's what got me into my latest project since I decided to serialize a novel because of it and I'm updating parts of it weekly on amazon. So that's content right there.
Other options include youtube. You could create your own channel if you are comfortable with it to talk about craft, etc. but if you aren't there is actually a THRIVING community on there called booktube with plenty of people willing to read/review books for their channel. Bookstagram is also a thing, which includes accounts on instagram solely dedicated to books.
Also, with Twitter, hashtags are key. Things like #amwriting or #freeebooks allow users talking about similar topics to find you. It's a great way to connect.
Besides which, if you're just starting out, be sure to check book bloggers. They're a great resource for reviews but also author interviews, giveaways, etc. Some book bloggers even allow advertising which gives you access to readers right away. Advertisements can run anywhere from $5 to $50 depending on the blogs popularity and size of the ad.
Also join goodreads and utilize that. Again, it's a huge resource for readers and a great place to get possible reviewers for your book. On the subject of reviewers: check similar books on amazon to see who has reviewed them and use that as a way to get reviews for your own product. Sorry if this is really long but I've been collecting a lot of info on the subject for my own benefit, so I thought I'd share my findings. -Stella