Warning: Cannot modify header information – Help!

I know this is a somewhat common, albeit, painfull issue that occurs, but when I try to visit my site and always when I try to go through WP Admin to access my site I get the following message:

Warning: Cannot modify header information – headers already sent by (output started at /home/content/58/5997758/html/wp-admin/index.php:2) in /home/content/58/5997758/html/wp-includes/pluggable.php on line 891

I scoured the internet, and although there are alot of posts on the subject, most give very vague responses on how to fix, or assume that you’re Bill Gates and instinctively know what to do with very little information. I know I have to look for ‘white spaces’ in the code, but how do I even get to that point if I can’t get through the WP Admin sign in page?

This problem showed up last week. I have done nothing to change the code in any way.

Please help! And talk to me like I’m 2 with this stuff. I’m a newbie. ??