Hi Ivan –
It sounds like your hosting provider has PHP error reporting turned up too high. These kinds of minor warnings are not harmful to your site, and they should not be appearing on the front end.
You may consider contacting your host to ask about a lower error level, but in the meantime you should be able to hide the warning messages by opening your wp-config.php file in your FTP client, and adding the following lines just above “/* That’s all, stop editing! Happy blogging */”:
ini_set( 'display_errors', 'Off' );
ini_set( 'error_reporting', E_NONE );