Search Engine Optimization (SEO) is a process of improving the volume and quality of traffic to a website from search engines through natural search results for targeted keywords.
Guide to Search Engine Optimization.

1. Make a website attractive to search engines. With a simple tag or key words, automatic results are in.
2. Manipulate visible keywords on your site.
3. Determine your web’s pages offer to visitors. It is important that you link such keywords in your site.
4. Create keywords or tags based on your site’s words.
5. Use between three (3) to five (5) keywords or tags on appropriate pages in your site. This is necessary to obtain full optimization of key words.
6. Remember to put or design your keywords or tags of your site from top left and down. Most people will surely view in the same manner. Search results should include the site’s title, headings, tags, abstracts and more.

Actually, you have the option to choose your keywords either before or after your site have been constructed. But I would suggest that you do it beforehand, it is easier, less stress because you don’t have to rewrite everything on the actual page.