Keyword research is the most important and also the most neglected step in website creation. A website can be perfected in everyway but is never useful if it cannot be found in the search engines. People use search engines alot and you want your website to rank well for certain terms. If you are in the web design business, you want your website to be in the first page in all major search engines (namely Google, Yahoo and MSN). Google is the king now, so you really want your website to rank well in google.
The sentence that you expect people to type into the search box to look for your website is your keyword. For example, if you teach people how to design websites and your keyword is "How To Design A Website", you should have different variations of the keyword in your website. Terms such as "How to, website design, web page creation, designing websites...etc" are good. One good keyword suggestion program that we often use is the yahoo-overture keyword selection tool. There are many such tools available, so feel free to look around.
Get a piece of paper, write down all the terms that is relevant to the theme of your website, ie your main keyword. Include all singular and purals.