To set correct permissions you need to use these commands:

chown www-data:www-data -R *          # Let apache be owner
find . -type d -exec chmod 755 {} \;  # Change directory permissions rwxr-xr-x
find . -type f -exec chmod 644 {} \;  # Change file permissions rw-r--r--

Depending on your server configuration you may put your wp-content on 775. This permission will allow your group to write in this folder. Why add group permissions? Because in wordpress, you can have two users working on files, the www-data user (who executes the website) and the ftp user (who downloads plugins and updates from the webplatform wordpress). You can put your wp-content on 755 but you have to make www-data the owner of this folder and do your updates manually via FTP.

You may be interested in this one too : http://codex.wordpress.org/Hardening_WordPress