Two concepts

1) On site optimisation

Make sure your content contains the keywords you need. This means the text on the page and also the page title. Put keywords in <h1> <h2> tags, consider using <strong> tags around keywords - use them in your internal links, avoid messy urls like the address of this page - ending ?mode=reply etc etc better to have keywords in urls. TO do this in ecommerce sites you need a url rewriter. Lots more, but mostly about adding lots of pages and content to your site.

2) off site optimisation - this tends to slowly improve over time as people link to you but it can take years, unless you provide a service or some content that people love and start spontaneously linking to.