The most important thing to know is that SEO is always changing, and some of the Google updates bring major changes causing things that used to work to now work against you, and even get your site penalized.
For example, directory submissions are not nearly as important as they were years ago, and you don't need them at all to rank well. Also, blog commenting can be considered spam and hurt you rather than help you. Backlinks are now more about quality than quantity.
So, if you want to rank well with proper SEO techniques, then it's vital to stay up to date and learn about what is currently important BEFORE you do anything that could potentially hurt your website ranking.