Do a little reading on Server Side Includes tonight and we will talk you through the process of
moving selected portions of your code to a separate text files which you can save as (for example)
header.inc, footer.inc, menu.inc
This will be the code for the parts of your page that will appear on all your pages.
Again for example, if your hosting supports PHP code you would past a small block of PHP code
into your page to replace the code you removed the code that is now in the .inc files
The small PHP code bits would look like
<?php require_once("header.inc"); ?> and
<?php require_once("footer.inc"); ?> etc.
Your pages would be named with a .php extension like index.php etc/
The server will merge the code from the .inc in the page where you have placed the small PHP code tags.
Any changes you need to make later in the .inc files will appear in all your pages at once
You need only one stylesheet which applies to all pages by inserting this code within the <head> section of every page
like <link href="styles.css" rel="stylesheet" type="text/css">
How many pages to do you anticipate having in your web site? If somewhere around 20 but less than 50, saving your prototype page as a DWTemplate (dwt file) might be the way to go.
As to CSS style sheets, one external sheet should be all you need for your site wide layout and text styles. You can use embedded CSS rules for page specific styling -- eg. you want to use a different background image on your contact page.