Hi Jion,
The best place to start is with an excellent overview on the topic to get yourself acclimated. Then you will need to go through the lists of free tools to use to help you do the work. (Also available on that site)
If you are using WordPress on your site, go to this site for a list of the best plugins to use that will greatly reduce the on page SEO time you'll need to spend. Excellent WP tutorial on onsite SEO.
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