SEO Optimization

Hello there, Well first of all let me say I can pretty much call my self an intermediate web developer and designer. I have a client that currently has a website that has been up for around 3 months now and hasn't had so much as 1 order placed on their site. I have pretty much no clue about how SEO optimization works and whatnot so I decided to post this project here. Basically they are an online store that sell women's fashion. They are running a modified version of oscommerce however no SEO optimization has been done to the store. I basically want a high Google rank so that they are shown in most of the searches for say "women's fashion", or "lady's tops" etc. And I would like some unique hits per day. Anything that will basically make users enter the site stay around and order something. So thats pretty much it for now.