On Page Optimization includes

Image Optimization
Css and Js Optimizaiton
Title Tag
Meta Tag
Sitemap.xml
robots.txt
Unique and Quality Content

On page SEO are:
Keyword Research
Title tag
H1 tag
Meta Description
Create Sitemap
URL Structure
Image Alt tag
Robots.txt
Quality Content
Headers tag

On Page SEO is one of the most important steps,

1. Meta Tag Creation
2. Content Optimization
3. Sitemap Creation and Adding
4. Robots Text file creation and adding
5. Internal Linking

Some of them are
Should focus in Title tag and Meta tag
Should concentrate in Content
do Image And Css Optimization
Sitemap.xml and robots.txt