Try using Google. This isn’t a WordPress specific issue.
You need to make a robots.txt file. You can start with this:
User-agent: *
Disallow: /cgi-bin
Disallow: /wp-admin
Disallow: /wp-includes
Disallow: /wp-content/plugins
Disallow: /wp-content/cache
Disallow: /wp-content/themes
Disallow: /trackback
Disallow: /comments
Disallow: /category/*/*
Disallow: /tag/
Disallow: */trackback
Disallow: */comments
…and then upload it to root folder of your website (where wp-admin, wp-content, and wp-includes folders are). After you upload it you can enter http://your_domain_name.com/robots.txt to see it in a browser or go to something like: http://www.spidertest.com/ to see if it works or not etc (I did not check this one, but I know it is not able to spider my site as I am not letting anybody in right now)
