The two important ones are keywords and description. They both go in the head of the document. Metadata help search engines find content relative to what was searched for. Let's look at this page: Keywords are important because they give the search engine an idea about the pages content. To create keywords, simply make a list of all the keywords you think people might find you with. Also, ask your customers to make a list of all the keywords they would try to find you with.
The description is a bio for the page. Try to keep it under a few sentences and related to the content. In either case, if you start repeating words, it can dilute the content and actually lower your page rank.

Search engines all use different algorithms to decide page rank. Metadata can help you improve page rank but, there are many other factors involved.

Perhaps the most important factor would be having other sites link to yours. It's similar to how rep power works on Dani Web:
If a person with a rep power of 8 adds to your rep it boosts your rep by 8 points. Now lets look at that from an SEO standpoint. If a site with a high page rank adds a link to your site, it will boost your page rank significantly. However if a person with a rep power of 1 adds to your rep it boosts your rep by only 1 point. So, a site with a low page rank adds a link to your site and it may or may not boost your site rank.

Above all, keep in mind that content is the best way to improve the site rank. If you have high quality content in your pages, search engines will rank you higher. Also, high quality content will also give other sites a reason to swap links with you, boosting your rank.