If you want
SEO avoid keyword-cramming like the plague; this means trying to put as many of your most common search terms into your content as possible. Many, many content writers do this out of a misunderstanding of the way google works; SE's don't care about quantity of keywords, the keywords just need to appear and the site needs to generate traffic. Hence, write naturally and engagingly - your site is for people, not the search engines.
Off-page
SEO is very different, and much more useful. Someone mentioned meta-tagging - read into it, it's brilliant. It allows you to use further, related keywords in the coding of your site so that SE's find it, but it doesn't annoy readers. Also, you may want to put alt-text to your images and videos (if you have them) google especially ranks sites with accessibility in mind much higher.
Next you're going to want to create backlinks, to raise your credibility - this means getting other websites to link to you. Blogs are good, forums are a little worse (they don't provide much authority) but are still good, but best of all are other functioning websites - entering into referral partnerships can be a good way to achieve this.
Hope that helps a little
Jordan