If you want to know how to SEO a Web site, you'll have to plan on doing some fairly in-depth research but we'll summarize the SEO process here.
So, how exactly does SEO work? SEO is a process and it's a process that should begin before your Web site is even started. However, you probably already have a Web site and it's never too late to optimise it. In fact, I suspect most sites don't get optimised until after they're launched so you're not alone.
Following is a high-level outline of the steps you need to complete to SEO your site for better search results.
Research your competitors' Web sites to see what keywords they're targeting.
Research the keywords to identify those that you think have the highest value.
Decide what keywords you want each of your Web pages to target.
Benchmark the performance of your site and your competitors' sites in all relevant targeted keyword searches.
Make sure the meta information for each page is done correctly.
Evaluate the text of each of your Web pages to see how effectively they respond to the target keywords.
Make sure that you have a page that targets each set of keywords or key phrases.
Make sure that the targeted keywords are used effectively in headings and that the headings are tagged correctly in the HTML.
Rewrite the content of each of your Web pages to target the keywords.
Check the architecture of your site to make sure that the CSS and HTML are designed properly to present the information in the correct order.
Set up your site with proper tracking tools like Google analytics and Google Webmaster tools.
Register your site with various search engines.
Get other sites linking back to your site.
Track the performance of your site in user visits and visitors who take the desired action like contacting your company or buying a product.
Evaluate the performance of your keyword optimizations and take corrective action as required. This is an on-going process that should take place at regular intervals - at least monthly or weekly.
If you think you would like to SEO your own Web site, you should read Can I do SEO Myself?