You can find out keywords in a number of ways. Look at your competitors' sites, look at previous marketing materials, look at industry websites, look at written reviews of your products aimed at your potential consumers/customers and any other written content around your product set or company.
If you sign up for services like Google's adwords they also give you the chance to review what words they recommend to consider as popular searches. AltaVista give you a refine your search results which can be helpful and Yahoo put things in categories. Spending time on things like this can give you ideas.
Once you have your keywords you must begin to use them effectively. You want them to appear in your text wherever possible but whilst making your pages still read normally. Like a viewer the search engines do not like repeated words and actually put your site down the listings. They don't mind the important words being in the page titles, paragraphs, image tags, meta tags but again also act reasonably. The most important are the first few paragraphs of text and the webpage title, as appears at the top of the screen for internet explorer and in the description on the taskbar.
Another reason to act reasonably is that more people will link to you and people will stay on your sites so that the other 2 main factors will be helped. The engines are becoming more complex almost human in their tastes and the number of workarounds is shrinking. Do your research and write good content and the search engines will respect your site.