Once you've built your site (preferably with a blog included) the best places to get involved are Twitter, Facebook, Google Plus, Linkedin, YouTube and Pinterest. Simply start contributing quality content and connecting sincerely and repeatedly with other users who share your interests. You will gradually attract direct website visits from those who see your URL listed in your accounts.
Remember also that the blogosphere can be seen as a vast social network in its own right. That's why you should participate in it with your own regularly updated, category-specific blog, as well as writing guest blog posts and lots of thoughtful comments on quality blogs in your niche.
As well as this direct benefit, there's a powerful indirect one: Google and the other search engines are eavesdropping on that conversation. They don't hear everything -- Twitter, Facebook and similar sites are often described as "walled gardens" after all. But the Big G catches a hell of a lot of it. That's why you want to "show off" your knowledge whenever possible on these networks (without big-noting yourself or overtly promoting your products and services, of course).
With much of what you've said, shown, written and shared in its memory, Google then has a very good idea of what you know and how much. And because you're always leaving your URL in these interchanges, the search giant associates your authority with it. The more good stuff you have out there, and the more it's shared by others in the form of retweets, likes etc, the higher you go up in Google's estimation. (This isn't the main factor it uses, of course. However the cumulative effect can be substantial.)
So there are two parallel processes occurring. And there's another one: Social media sites are teeming with bloggers and webmasters. So if you can impress them with your expertise and knowledge in your field they are much more likely to link to your URL from their blogs and websites. These one way voluntary links from trusted, relevant sources are very powerful in SEO terms. Get just a few of them and you're sure to start crawling up the rankings.
If you do this you don't really have to worry too much about search engine optimization. It will develop organically. You are absolutely certain to draw substantial, quality traffic over the long term. (Sure, you might not end up ranking highly for the exact keyword phrases you want. And you might not get there as quickly as you'd like. But then you can't have everything, can you?)