Errors appear for some reason

These errors have appeared for some reason and I don't know why. Can anyone help please:-

Notice: Undefined index: HTTPS_HOST in /home/mydreams/public_html/onlinesitehub.com/bandbytes/index.php on line 51
Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/mydreams/public_html/onlinesitehub.com/bandbytes/index.php:51) in /home/mydreams/public_html/onlinesitehub.com/bandbytes/system/library/session.php on line 12Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/mydreams/public_html/onlinesitehub.com/bandbytes/index.php:51) in /home/mydreams/public_html/onlinesitehub.com/bandbytes/system/library/session.php on line 12Warning: Cannot modify header information - headers already sent by (output started at /home/mydreams/public_html/onlinesitehub.com/bandbytes/index.php:51) in /home/mydreams/public_html/onlinesitehub.com/bandbytes/index.php on line 179Warning: Cannot modify header information - headers already sent by (output started at /home/mydreams/public_html/onlinesitehub.com/bandbytes/index.php:51) in /home/mydreams/public_html/onlinesitehub.com/bandbytes/system/library/currency.php on line 45

What exact version are you using?