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

This is important for two reasons:

1. Search engines need to know what your site is about (obviously) and the title tags give them an important clue.
   
   
2. The title tag will show up in the search results. If people don't click on it (i.e. a low 'click-through-rate') search engines will punish your site, because they aim to display only the most relevant content to users, and if users don't click on it, search engines see that as an indicator that your content is not relevant - and your ranking goes down.
   
   

See a detailed description of how to give your websites proper title tags on MOZ. 

Make sure that the content you own is not hidden in some databases where it is only accessible after users entered a search request. Put the content online so that search engines can crawl and index it, and subsequently send traffic to your site. 

One example for this is Facebook: user profiles used to be "behind the curtain", i.e. only accessible to users who are logged in. More and more content is now publicly accessible - for exactly this reason. 

Great content will pay off. Google and other search engines are now doing a great job in punishing websites which try to rank high with "black hat" techniques, but they also reward sites which do create great content.

Maybe you don't have the right regional focus? Maybe your concept/idea works better in a different country? 

A tool which allows you to easily offer your site in 90 languages is Google Translate. 

Note: this is also a good strategy for SEO (Search engine optimization) as you'll have content then in many different languages - without putting any effort into it. 

Search engines love content. A lot of content = a lot of traffic. But how to offer unique content on your website without stealing, and without spending a lot of time (or money) to create some? 

Fortunately, sometimes there is free content out there which is up for grabs. Frequently organizations or institutions who are not internet-savvy don't know that the content they have could be put online (or made directly accessible to search engines, i.e. not hidden in a database). 

For example, imagine you're a new website for lawyers. What information might be out there which is not directly accessible on websites today? Maybe there is some government database with tons lawsuits, with a description of the lawsuit and the outcome etc. Maybe it's even accessible via the web, but only through a search form (so search engines don't crawl that data). Approach the governmental body in charge and ask them if you can use the data and make it accessible to users in a userfriendly form. Maybe they agree to it - why shouldn't they? They may also have an interest in their content getting found.

Search engines track which websites get updated and which don't, and reward / punish them accordingly in the rankings.

Therefore make sure new content gets added to your website(s). It doesn't have to come from you - if you develop a good strategy which makes your users / visitors provide comments then it also does the job (and you don't have any work). 

See a detailed description of this on SearchEngineWatch

Although search engines only understand text, they do recognize the existence videos and pictures, and often regard a mix of several different content types as possibly more informative than text-only, and reward it accordingly in the rankings. 

Links to other (high-quality) sites signals that you are (or at least consider yourself to be) in a good neighborhood, which is usually rewarded by search engines. 

However, if you link too much search engines may see this as a sign that you don't have a lot of relevant content on your site (that's why you have to send your visitors to many other sites) so your ranking will suffer. 

One way search engines assess the quality of your site is by how long people stay there. If visitors bounce very shortly after they came to your site (e.g. from a search result page) then search engines see this as an indicator of low quality content on your site.

Vice versa, if you succeed to engage people and make them browse many of your sites (not only the 'landing page' where they entered) then that wil have a positive effect on your rankings. 

The H1, H2, H3 tags tell search engines something about the structure of your page (i.e. headings and sub-headings) and are therefore a must-have in on-site-optimization. 

The video below gives a good introduction: 

You've probably seen it before: some websites have more information displayed in search results than only the title and snippet of their page. For example, some have a picture, or a review (1-5 stars). 

This additional information is called "Rich snippets" and it increases conversion significantly. You only need to tweak your pages slightly to have those displayed. Read more about it on SearchEngineLand.

Https is a quality indicator for google and other search engines. So switch to https, buying a security certificate is cheap these days.