Re: Simple question, Im guessing there is a very easy answer!
/wp-content//themes/my-theme/header.php 
But what do you mean by ‘put something’? there are various means of adding passing to this file and on to rendered output, yes you can directly edit it though (in a child theme.) but it might be advisable to just state what you are wanting to do in case there is a specific and proper means of doing it.
