Some people would have you believe that to load your page with as many keywords as possible is the way to go. I personally prefer to use up to around a dozen carefully selected words, with a couple of conditions.
Keywords, to work properly, have to be mirrored in the page text somewhere. On their own they will help but if you select carefully chosen keywords that are also present in the body text copy it will go a lot further for you in search indexing.
The problems with metatags are many and I'd need to type pages and pages to cover all the bases. Suffice to say that a combination of a few well thought out keywords that are reflected in the body text will work far better than a long list of vaguely related words. It's all about striking a balance between page titles, keywords and the page content and each page in the site should be treated differently.
Try to choose your keywords based on a potential site visitors likely search requests. Keywords are weighted with various 'strengths' based on how valuable they would be in finding your site and its content from the many others out there.
If you want some links to sites that cover this in more detail just ask, but don't get too lost in it unless search engine optimisation is right at the fore of the particular site you have in mind. Metatags are no substitute for including the site URL on all business literature and correspondence and this, for small businesses, is the easiest way to get your site noticed in the short term.