SEO is about optimising the site contents so it has good structure and is readable for visitor and google bot.
The SEO tools can help get an idea on what to improve on the website.
Good SEO tools:
Webmaster Tools - https://www.google.com/webmasters/
(add your site and get targeted advices, analyse your site)
Google Analytics - http://www.google.com/analytics/
(one of the best visitor counter and website visitor analytics)
Google Trends - https://www.google.com/trends/explore
(see the Google search volume on certain search prasses, get ideas)
Keyword planner/Keyword Tool - https://adwords.google.com/KeywordPlanner
(discover interesting phrasses which people search, gives you ideas on your site SEO and new content)