Off-page SEO is as important or even more important than on-page SEO, and the main thing you should concentrate on when maximizing off-page optimization is building backlinks. It will not only drive more traffic to your website, but it will also establish the authority for your website.
When Google Robots see that a link to your website is placed on a very respectable and popular website with high Page Rank and high traffic, it will make your website rank higher as well. And the more backlinks (the links to your website from other websites) you have, the better it is for your page rank.
I am not going into details on how to build backlinks, because it is not my area of expertise, but I think you already should understand the basic principle of how it works. Just remember that places like social media or forums are not providing as much quality and authority as big independent websites, although links from these types of websites are also helpful.
A few ideas on how to build backlinks:
Create infographics, and share them with these websites.
For bloggers, guest blogging can be very useful. You provide valuable content and link it back to your website.
You can also analyze your competitors and see what websites link to their web address.