I have websites that needs an SEO expert to make them on the top of Google searching result based on some keywords, or at least the first page, the websites are created using common open source platforms (wp, joomla, or magento, zencart)

I want to post a job on oDesk but I have no idea on how this particular type of requirement works? what would the guy do on my website? will I be asked to spent extra advertisement money? How do I know s/he really did anything on my site and what have been done? if it's just to optimize URL to make it meaningful then I don't need that, and I am sure giving every page's URL an alias is not enough to make the site popular.

Beside, I want to know how much it roughly costs and how long it takes.

It would be great if you can shed me some light, thank you in advance.