Opencart Demo website Home Page Error

Hi

I have just started to use opencart since 3-4 days. My demo website's homepage header reads something like below -

Notice: Undefined variable: _SERVER in /home/apack996/public_html/system/startup.php on line 28
Notice: Undefined variable: _REQUEST in /home/apack996/public_html/system/library/request.php on line 26Warning: Cannot modify header information - headers already sent by (output started at /home/apack996/public_html/system/startup.php:28) in /home/apack996/public_html/catalog/controller/startup/session.php on line 25Warning: Cannot modify header information - headers already sent by (output started at /home/apack996/public_html/system/startup.php:28) in /home/apack996/public_html/catalog/controller/startup/startup.php on line 99Warning: Cannot modify header information - headers already sent by (output started at /home/apack996/public_html/system/startup.php:28) in /home/apack996/public_html/catalog/controller/startup/startup.php on line 157


Please assist.

Thank you

Regards,
Aliakbar