Modifying the Header.

I want to add the phone number to the right of the logo in the header. I am using a child theme. What file do I go into do add some HTML. I am very good at HTML, CSS, and Php. I am a WordPress developer. I thought I’d ask you first where to do this and how would I integrate that into the child theme structure. I don’t want to add it in the above header space.

http://gyazo.com/0cbcfd5d586c6b285340a0a47379c628

Maybe something like this http://gyazo.com/50cf351e3abe621986d5cb6521d051b8

If you tell me where, I’ll add the HTML and do the CSS. I’ll hide it with a media query for mobile because I know thats where you put the mobile nav menu. Thank you!