The best advice I can give is to add percentages instead of fixed widths….unless you know how to override those fixed widths in the responsive section when the browser is resized. So the easiest way to go is to use percentages…but this may or may not work depending on your setup and what you want. Fixed widths will be harder as more skill is needed to override that code in your responsive section.
If you can’t figure it out, I’d recommend searching for a tutorial on responsive design/code (there should be a lot of info out there).