What file to edit for the horizontal menu bar

[masism]

I'm trying to change some things around on the top horizontal menu bar but can't seem to find anywhere to do that. Any help would be greatly appreciated. I'm new to all this so I'm missing some of the fundamentals, I'm having a hard time just figuring out where to look to edit the code at ex. editorial.tpl, header.tpl, global.css, etc. Once I figure out where it should go I can go move along.

Another question I have is when I look at the source of a webpage (right click view source) can this html be edited and where can I find it in my bo to edit?

Thank you