You need to start working your way through these resources:
How to clean and fix hacked WP blog:
http://codex.wordpress.org/FAQ_My_site_was_hacked
http://wordpress.org/support/topic/268083#post-1065779
http://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
http://ottopress.com/2009/hacked-wordpress-backdoors/
http://www.jtpratt.com/how-to-fix-a-hacked-wordpress-blog/
http://sakinshrestha.com/wordpress/fix-if-your-wordpress-site-is-hacked/
http://www.wpbeginner.com/wp-tutorials/how-to-find-a-backdoor-in-a-hacked-wordpress-site-and-fix-it/
Harden your WP installation: http://codex.wordpress.org/Hardening_WordPress
Additional Resources:
http://sitecheck.sucuri.net/scanner/
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html