Most likely you did not't find what you were looking for. If you regularly use the Web, you soon learn to narrow down your search by using groups of words, and by entering specific words relating to whatever you're looking for. People use search engines to look for specific information, so they choose keywords which narrow down their search to exactly what they're looking for. If they're not, then they probably will not find what they're looking for and they certainly will not find your site. If your Web site provides information or sells a product which has not already been 'done to death' on the Web, then you've won half the battle already.
Some search engines ignore words used very frequently on the Web.
This saves storage space and processing power. Don't choose these words as your preferred keywords. Cross them off your list. If you're not sure if a word is too commonly used, try searching for it. If your search turns up more than 2-4 million hits, then cross the word off your list.