most relevant variables in SE visibility?

[DabliW7]

I am new to web design, web promotion, etc., and I'm only at it because we are a small business just come to life and I need to do things by myself. So: I read whatever I could find about what actually determines SE retreiving, ranking, etc. From meta-tags, tag placement, url name to submission strategy. But I haven't come accross any quantitative study or even just a statement about what is most relevant to ranking. I am sure SEO companies work precisely on that to asure that their costumers get listed and ranked sooner and higher.
Any references or info is welcome.