I don't have a server to put it on...
...I made them php files instead of html for when I eventually get around to learning that, but it doesn't seem to be working on my browser at the moment
PHP is a server-side language, you need to install a web server and PHP (or rent one from a hosting service) and run the php files through that
As far as your actual content, I do have some critique:
First thing you should know, is that the <head> tag is not to be confused with <header>
<head> is for metadata and stuff like <title> only, anything that should be displayed on page (like all your links) should be within the <body>. Browsers may choose to render it anyways, but they're not required to as it is invalid. wait derp i was confusing <link> with <a>
Second thing you should look for is some sort of template system, so you don't have to do things like copy and paste the same header into every page. ASP.NET has master pages, I don't know what PHP offers.
Third is more minor, but HTML5 added a
bunch of tags such as <nav> and <content> that effectively replace <div id="nav"> and <div id="content">. It also deprecated tags like <center> in favor of CSS (text-align:center)
