Keywords are one of the most important parts of making your site visible.    Keywords are terms or words that are used by the search engine to retrieve sites that have to do with the keyword entered. 

One thing that you will want to do is pick keywords that have to do something with your site.  You will want people to come and look at your site and want to information about what you have.  Not wrong keywords.  Since most people want relevant hits, you will want keywords that are you close to what you have on your site. 

If you have a site about genealogy, you might want to use genealogy and synonyms and Related terms.  In addition, you might want to use words close to what you have on your site.  One of the best things to do is look some of your keywords in a thesaurus and then list all of the ones that you have in the thesaurus. 

You want to be sure that you use both the singular and the plural version of the keywords.  A person-seeking geologist might also type the word geologists.  You would want the person to find you site either way.  It is ok to repeat your keywords but not excessively.

Many search engines will rank your site higher if it contains multiple instances of a word that is searched.  However, currently some of the major search engines have been rejecting your site if you repeat the keyword more than two or 3 times. 

If your keywords can be punctuated differently such as NASA then you would want to also use it N.A.S.A . and put it in lower case.  Then you can get more keywords and you search engines do matter on the case of the letters.