SEO 101
First things first, What IS Search Engine Optimization or SEO? Well most of the readers wouldn't be here if they didn't know this. But still there are lots of commercial sites out there designed by people with no knowledge of SEO. So here it is:
Search engine optimization (SEO) is a set of methods aimed at improving the ranking of a website in search engine listings, and could be considered a subset of search engine marketing. - Wikipedia

Okay the first thing you need to know is ... Search engines index ONLY TEXT. No images.. No Flash. Well now Google says they have developed an SWF parser to index data from Flash content. And search engines are becoming more intelligent and now index PDF files and also captions of image files. But the ONLY TEXT is a very useful rule.

The key thing is that you cannot just add a magic code to your site and POOF!! Your site is first in Google listings. Okay seriously, there are many things you can do to improve your site's ranking. Every little bit helps.
It cannot do if you optimize just one of your pages. Do it on many pages. Don't bother with pages with no real content like the Contact form etc. Optimize those pages which the people are searching for. Like your Main page.. Your product listing, your article etc.

The META tags
These tags are invisible to the visitor but are read by the search engine bots.These used to be important sometime ago like the keyword tag for example. But now it is mostly ignored due to abuse by spammers. The most important ones are: