WordPress on page SEO requires the same steps as any other sort of site but in addition needs a few changes to the standard configuration plus the use of a good theme and various plugins.
On Page SEO for all types of sites includes:
Selecting good keywords - this is the basis of all SEO and where you can fail before you even set-up a website.
Getting a good domain name - this should include the primary keyword. If you can get a clean aged domain that is suitable, that would be the ideal.
Adding unique content - you site will be boosted by having content that is not available elsewhere.
Arranging each page around a keyword - use the keyword in the URL, title, headings and text whilst keeping the content readable for visitors. Do not overdo it as this could be seen as keyword stuffing which the search engines do not like.
Linking - link all your pages together using the keywords as anchor text. Also link out to relevant authority sites, preferably from within your content. Do not link to any dubious sites.
Including "legal" pages - legal policies and About and Contact pages are essential for on page SEO purposes.
Frequently adding content - search engines like to see that your site is active so frequent updates are good.
Making a site map - ensure there is a site map for the search engines (WordPress users can install a plugin to do this).
Additional things to consider for WordPress on page SEO:
WordPress Theme - some themes are designed to look nice without considering on page SEO. Use one that was designed with SEO in mind.
Permalinks - Change to "postname" so that the page/post title appears in the URL rather than a number.
SEO Plugin - install a SEO plugin such as "All In One SEO Pack". With its default settings this automatically improves on page SEO by, amongst other things, preventing search engines indexing duplicate content, providing the ability to enter meta data for the site and each page post and giving control of the title formats (you might want to remove the blog name).
You can also install a different type of SEO Plugin which is designed to check your on page SEO. The SEOPressor plugin analyses each page and post and gives it a score for on page SEO along with suggestions for improvements.