SEO is a very complex topic. It takes many hours of reading, experimenting, and analyzing to become proficient. The hardest part about learning online is figuring out who to trust. If I had to pick four free resources from which to learn SEO I would choose the following:
1. Google Webmaster Guidelines – This is a pretty comprehensive document outlining how to do SEO the Google approved way. If you were to stick with only following the rules in this guide your site would be well ahead of much of the competition.
2. Yahoo Search Topics – Particularly the section which discusses their search spider called Slurp and the section about ranking. It also gives you some information about how to use Site Explorer to look up backlinks, something all SEO’s should learn how to do.
3. Bing Webmaster Center – Again, pretty similar to the other two where it’s set up into different sections related to technical recommendations, content guidelines, and things to avoid.
Those three are very similar, but if you read through them you will spot similarities and come away with a good understanding of what the search engines deem important.
4. Once you’ve conquered those three help centers, you’ll want to make your way over to Webmaster World’s Google Hot Topics section. This is far more advanced than the other three but provides great insight into the intricacies of creating a succesful site and ranking well in Google. Definitely a little more advanced than the three listed above, but this resource is well worth the read. Definitely a place to come to if you have questions about your site or how Google may be treating your site.
Bonus:
5. Techniques and Failures for Web Content Accessibility Guidelines 2.0 – Digesting all of this content would take months or years. It’s not important to know everything on here, but the more items from this list that you implement within your site the better off your site will be from a conversion aspect and an accessibility aspect.