Wow, that's a very broad question. Of course you could spend a million dollars on online courses, or spend a million hours reading blogs and websites. SEO is a hot topic, so watch out for people trying to sell you the latest ebook or webinar.
There are really only a handful of rules for SEO, and if you start with the basics as you learn more you can tweak your techniques. First off, write a lot on a small number of topics. Update your website at least once a week, more often if you can. Keep adding fresh content until your search engine rank is where you want it to be, and then don't stop.
Use free tools to research good keywords, like the Google Keywords tool. Copy and paste this address into your browser:
https://adwords.google.com/o/Targeting/ ... earch.none
Choose only 4-6 keywords and phrases and include those in every article you write. Also include your physical location (My Physical Therapy Clinic in Frozen Tundra, Minnesota) so local people can find you. "Keyword density" is a complete myth. Just use your keywords naturally as you write and Google will see you. Use too many keywords and your potential clients will leave your site in droves.
Go to this blog and check a top 10 list of SEO blogs:
http://www.seomoz.org/blog/best-seo-blo ... y-uptodate
Don't get overwhelmed. SEO isn't the big mystery all the experts say it is. Be patient. You can't raise your rankings in 24 hours. (Don't buy the ebook that says you can.) I don't know whether you're a writer here or a customer looking for advice. If you aren't planning to write and submit to Constant-Content, maybe you'd like to get some articles from one of us? If you request they be optimized, they'll be optimized.
Let us know if you need anything else--we love to answer questions around here. Have a good day.