First thing to do is search here for template_default and read about all the reasons that you should create a template based on the template_default fileset. You are curently using the classic template and this uses 2 stylesheets and will be problematic now and later.
It appears that you have solved this
How do you change the alt message on the logo.gif?
Also solved this
How do you change the "sales message goes here" message?
Once you have created the template refered to above, open the stylesheet.css file in a text editor of your choice and find this code:
How do you use the full width of the top part of the page to add a full width banner? (bet that doesn't even make sense!).
change the red highlighted value to the width in px or % that you want. If you do not have a "fluid" type of header image set this to the value of the width of your header image.
/*wrappers - page or section containers*/