Hi Pam, the link Kate posted in a superb place to start learning about these things. As is the
Search Engine Optimization (SEO) section of the forums.
To answer your question more directly - I think that you should include whatever meta tags best describe the site or page you're writing them for - you want to think about what people will be typing in to find you and your services. But don't include too many.
And really, you want to focus on your actual content more than things like tags - Google does, and so do your visitors. Meta tags and similar are secondary, so spend that extra time writing great, well optimized text on each of your pages and treat meta tags like the 'frosting' that they are
Hope that helps a bit!