I’m just beginning and struggling with this myself.
I’m using two plugins:
1.”All in one SEO pack”
This does a number of things, chiefly making it easy to create meta tags for pages.
2. “Google XML Sitemaps”
This is one thing that the above plugin does not do, it creates a sitemap in the form of an XML file which helps search engine’s web crawlers.
In addition CSS and XHTML markup validation are important. Alot of that can be done easily by using proper tags in your posts. See this page: http://codex.wordpress.org/HTML_to_XHTML
Plus SEO is not something you can add onto a site. It needs to built into the design and the site’s content right from the start.
Have a read of Search_Engine_Optimization_for_WordPress
