Pitafi - I get the sense from the way you're asking the question that you may be asking about adding meta keywords to a site that's already been built. If so, be aware that the meta keywords tag is essentially useless for ranking.
Meta information is not trusted by any of the major search engines... largely because it is so misused. Instead, the engines tend to reward vocabulary that's prominent on the page (but not so prominent that it's spammy).
The best time to choose search phrases to target and related vocabulary to use on a site and on a page is before you build the site or the page.
But late is better than never.
Though I don't agree with all his comments, Dennis John offers some good starting suggestions above. You need to put yourself in the shoes of your customers, determine how they would look for what your site offers, and incorporate that vocabulary into your visible html text content, and into the title element on each page. Then, research that vocabulary exhaustively.
Note that search targeting is predominantly page specific, so don't try to get everything into one page.