Installation help please.

The easiest way is to take one of the two default themes and modify them to include your current header etc. But the only thing you really need in it is the loop.

https://www.ads-software.com/support/topic/25522?replies=24

Just search Codex and the forum, there are plenty of guides that will guide you. Including:
https://codex.www.ads-software.com/User:Jalenack/Integrating_Wordpress_into_an_Existing_Site

This tutorial may help you: https://www.transycan.net/blogtest/2005/07/05/integrate/

And some other info:
https://www.ads-software.com/search/integrate+wordpress+existing+website?forums=1

Then you have to make a theme that looks like your site.
(before that make sure you do something about the width of your existing website: anything below 1374px causes horizontal scroll)
https://codex.www.ads-software.com/Theme_Development

The easiest way is to take one of the two default themes
I’d never say something like that for a newbie… on the contrary: stay away from the “Default”!

I’d never say something like that for a newbie… on the contrary: stay away from the “Default”!

I created my original WordPress theme by modifying the default themes. I found it far easier to replace a few divs than to completely learn the themeing process.