Headers already sent problem

I have a website that is name

Code:

http://www.example.com/

i noticed that when i type in the url box

Code:

http://www.example.com/

it gives me an error which is

Code:

Warning: Cannot modify header information - headers already sent by (output started at /home/ghany1/public_html/wp-blog-header.php:1) in /home/ghany1/public_html/wp-includes/pluggable.php on line 122

But when i type

Code:

https://example.com/

in the url box without the www. it works just fine

So i decided to check the forums about similar issues and i found it some how related to spaces but i couldn't find any extra spaces to delete so i played around and found out that in the 1st line in blog header.php there is a encode base 64 with a code beside it

Code:

<?php echo base64_decode("there is a very big code over here=");
/**
* Loads the WordPress environment and template.
*
* @package WordPress
*/

if ( !isset($wp_did_header) ) {

$wp_did_header = true;

require_once( dirname(__FILE__) . '/wp-load.php' );

wp();

require_once( ABSPATH . WPINC . '/template-loader.php' );
}

Then i deleted the whole line incline the base code and stuff and clicked on save changes luckily the website worked on the www. but when i woke up the next morning it still gave me the same error and when i checked it again it generated a whole new different code ? can u plz tell me if this code is important or nope and how i can fix the www. problem and if the fix is from deleting the code how can i stop it from generating another one