The general settings regarding SEO can be
found in Settings -> Store Settings -> SEO settings on the left
hand side. There you can set up the default meta tags, verify your site
with Google or Yahoo. You can also set up the way meta tags are
generated on your product and category pages.
The rest of the work with meta tags needs to be done on your products,
categories or brands. Each one you create has a description and keywords
fields that can be used to customise your meta tags.
We strongly advise to get a professional to work on your SEO if you do
not have experience with those matters. Improper SEO setup might hurt
your site rankings.