Sorry for such a generic title, but that's really what this is about. Here is an example of a page you can check to understand the changes I'd like to make:
Here's the screen shot with the explanation of these changes (marked and written in red):
*Check attached file.
Some of them apply to my header, others to my products' pages, as you can see in the screen shot -the page shown there is just an example of a product's page.