It's a moving target with enough books written on it to fill a library, all out of date by the time they hit the bookshelves.
Visit
Web Monkey for free advice and tutorials on all things web and for links to other great resources.
For SEO and SE registration, articles, tutorials and actually doing it, go to
Tooter and browse a while. Join. It's free for the first month, then you decide how much to pay (he suggests $10 per year, but it's up to you). I've added my affiliate code, which is easy enough to remove. This guy does his research and stays up to date.
The other great SE learning resource is Google. If you have an Adwords account, see its very useful baggage of tips and tutorials. If not, go to sign up for anything with them and follow the links instead.
AVOID forums for SEO until you have far more experience. They are full of smoke, mirrors, rumours, superstition and just plain bad advice which you need to be able to filter out.