Benoit, I am going to do you a favour. Read this:
http://www.seomoz.org/articles/beginners-1-page.php
Read it, read it again, then, when you're done, read it again another time, just for good measure. It won't make you an expert in SEO, but it will teach you the basics.
One final thing. Keywords and keyword density do matter, but they matter in the visible parts of the page (text, headings, title) and backlinks, not in the "keywords" meta tag. The major search engines figured out long ago that meta tags contain what the website owner wants to rank high for, not what the page actually contains. People spamming meta tags with keywords pretty much killed the relevancy of meta tags for the major search engines.
PS. You may want to consider getting a new username.