Have you logged into your host via FTP (any FTP client will work or use the hosting control panel) and found the robots.txt file? If so, edit that file and remove the lines of text you want removed.
As a rule, I wouldn't block the /plugins/ folder as this may trigger a problem Google looking for CSS or Java. I also wouldn't block the /wp-admin/ path - there's other ways to prevent hackers.
I will block paths to affiliate links or downloads. And I always include a path to the sitemap.
Hope that helps!