Search engine optimisation is commonly divided into two main groups: in situ and external optimization. To achieve good classifications, you can not have one without the other, but this article is mainly related to in situ optimization. These are factors that anyone with a basic level of website understanding can manipulate and improve in an attempt to show the search engines that their site is worth visiting.
Page titles are one of the most important in situ factors in search engine algorithms. Search engines see them as a guide for a page topic, so they must be rich in keywords and be specific to the content of the page. This is where good information architecture (site design) will help. Ideally, each topic within your website should have its own page, allowing you to optimize a page for a small set of key phrases. Search engines strive to deliver web pages to users that are directly related to your search term, so the more specific your web pages are for a search term, the more likely you are to get good rankings.
Body content must also be defined and related to the subject. Good writing is a skill that will only come with practice, so go ahead! Make sure your content is well read and contains numerous mentions of the key phrases you are optimizing, although be careful, do not make your content illegible due to frequent mentions of key phrases. This will only reduce the quality of your website and the search engines will realize this and will probably penalize your website. It is also likely to lead to a high bounce rate.
URLs are an important factor in on-site SEO. It is vital that they include key phrases related to the content of the page. They should be as short as possible, especially if your site contains many different levels and subsections. They are given more importance to the characters and, therefore, to the key phrases that are near the front of the URLs, so this must be taken into account when designing the architecture of a site.
Headline tags can be used to include key phrase variations as pluralizations. They indicate the subject of the page sections, so the search engines consider it a good indicator of the key phrases that are selected.
All of these factors are important for in situ optimization. If you push the time, I suggest starting with page titles and URLs, since they are given more importance in the search engine algorithms, but all the factors must be considered and optimized if you want to achieve the best rankings for the most competitive key phrases.