I need to have my site visitors post comments even if they are not logged in as a member
You need to give the "anonymous user" role permissions to post comments. Go to admin/people/permissions and look for the Comment section. Check the box for "Post comments" for the Anonymous user role, then save the settings.

I would not suggest this unless you have good anti-spam measures in place. You can search drupal.org for modules in the Spam Prevention category to help with that. Otherwise, I would require users to sign in before posting comments.

Note that even when requiring users to sign in to post comments, you'll still likely run into spam. Couldn't hurt to look into the list of modules linked to above.