1. Create a template.html page, keeping the styles & scripts; just no unique page content.
The next step I am unsure of based on your tags not quite aligning with your post. You stated that you are an "asp programmer" but you have the post tagged as "ASP.NET" and "WebForms".
If you are going the ASP.NET WebForms route, you would proceed with
2. Create a master page based on that template.
Creating a Site-Wide Layout Using Master Pages (C#) | Microsoft Docs
If you are going the Classic ASP
route, it will be a little more difficult as the tutorials will be harder to find. You can start with ASP Tutorial
What you would need to do is to "cut up" the pages into section and create individual files for them (eg: PageHeader, PageContent, PageFooter).
You would then create a new template- I'll call it template.asp. This page would have
directives to call those sections in the correct place.
To create a new page; simply copy over the template and fill in the gaps between the includes