What is the most important part of on page seo. actually i didn't have experience in on-page seo.
does any one can suggest me any resource/site that can help me to learn on page seo perfectly..
Google offers plenty of advice on all aspects of SEO. I would recommend starting by downloading their Search Engine Optimization Starter Guide from this page
You might also want to search the forums, as there have been other threads on similar topics, such as http://www.sitepoint.com/forums/showthread.php?902449-SEO-Friendly-Website-Design&highlight=onpage+optimization.
In my opinion? Great content and good coding practices (that includes addressing accessibility issues). That's the best SEO you can ever have...
Obviously, once you've done a great site with great content, it helps to then do a bit a promotion (which may not be SEO-related) but most of the times, consistancy and good content makes the miracle and backlinks start to come by themselves, and traffic adn all that.
But @TechnoBear;'s links are good too
Search Engine Land
Search Engine Watch
You can find seo beginners guide in the above mentioned 5 websites.
In summary: have the right keywords in title, meta description, headlines and paragraphs