My website doesn't rank high in search engines ((Only) Off−page optimizations)

How many sites (and esp. how many high-quality sites) link to your site is a key factor which determines your ranking in google and other search engines. Therefore you should always seek to get high-quality links to your site. 

There are various strategies to achieve this. A well-structured longlist can be found here.

Just like you (should), other websites are constantly seeking to get unique content on their site, to increase their readership and enhance their ranking in search engines. Therefore many allow guest posting, i.e. where guests provide an article and are permitted to place one or two links back to their site in return. 

A service for which guest blogging made all the difference is Buffer (read their story here). 

Search for high-ranking websites in your niche and ask them if you could write a guest post for them. Not all accept guest posts - you'll see that if you check who is posting on their site. A longlist of high-traffic sites which allow guest posting can be found here. 

If you manage to get links to your site, make sure that they are descriptive, e.g. a site linking to you with the anchor text (= text in the link) such as "Check out this great site to find solutions to all your problems" is not as valuable as "Check out Solutionbay, the no.1 site to find solutions to all your problems."

Getting links from blogs-only (as an example...) is by far not as powerful as getting a mix of links from blogs, newspapers, social media and so on. So try to get a diversified link profile. 

Search engines are getting smarter every day... and they figured out a long time already that a link from the content area of a site is a much more powerful statement than a link from the footer.

So try to get your links into the heart of the sites' content which link to you.