While there are lots of good tools out there that can help you to gather information and provide some pointers, there is no replacement for a thorough understanding of current SEO practices.
When all is said and done, one of the most effective things you can do is to build an attractive site with good content that delivers a good user experience and therefore holds the user's attention (easier said than done I know lol). User factors/signals are said to have a massive influence upon SEO since the leading search engines are able to spot sites that perform well. For example, if you have a page that visitors land on and immediately hit the back button on their browser, this is a strong signal to the search engine that the visitor certainly didn't find what they were looking for on the page and that the page may therefore not be relevant on that SEPR (search engine results page). Ideally you want the visitor to terminate their search when they arrive on your site.
You can read Google's guidelines at:
is another useful scanner you can use to make technical optimisations and avoid mistakes.