Cannot modify header information – headers already sent by

Hi ,
I have updated wordpress and always i am getting such error while opening wp-admin.

“Warning: Cannot modify header information – headers already sent by /wp-includes/functions.php:5”

Can anyone help me how to resolve this issue.

Also please help me how to secure wordpress site because i am seeing nw a days its getting hacked easily & they keep adding some eval base64_decode value