The basics to creating good meta tags
It is best to plot out how you’ll use meta tags on a particular page before even writing the content for the page. Lots of people will write the content of the page and then try to sprinkle in keywords in the meta tags. This is an ineffective way of doing things, as it usually makes your page confusing to visitors.

http://marketersadvertisinggroup.com/marketing_seo.htm

Before writing content, take a minute to pick out 3-4 primary keywords. Then take another minute to write phrases from those words. Once you’ve done that, you can begin writing your content around the keywords.

How do I select proper keywords to enter into the meta tags?
This is an excellent question. Really, there are two main rules of thumb for selecting proper keywords.
1: Base your keywords off of the content of your site.
For instance, if you are running a digital photography site, your keywords could be digital, photographs, photography etc. You’ll want to steer clear of silly keywords that don’t relate to your site. These would include keywords completely irrelevant to your site.
2: Make sure your keywords can flow naturally throughout the content of your site.
Try to pick words that you can easily incorporate into the content of your site and that make sense. You must remember that actual people are reading your site-the content(and keywords) MUST make sense to not only search engines, but also to the people who visit your site. Thus, the keywords you choose must fit the content of your site perfectly.