Glenda,
Go to Site > Global Configuration (assuming you're an administrator or super administrator). On the site tab, you'll see a section for Metadata settings.
Fill in the description and keywords fields and click save.
For the description, it's recommended to keep it under 70 words (around 350 characters). Keywords are not so much used by google and other search engines any more, but are looked at at the article level by the internal joomla search.
To add metadata to each article - recommended so that you don't have the same global description on every page - there's metadata parameters on the right of the article editor screen.
Patrick