Don't use "<center>"~"</center>". This is deprecated (old, out-of-date) and is you use a STRICT !doctype, you'll throw errors.
Surround your page with a DIV. Let's call this a 'wrapper'.
<div style="width:90%; margin:0 auto;"><!-- wraps entire page content from opening BODY tag, to closing BODY tag -->
-Your site content goes in here-
</div><!-- end WRAPPER -->
This works, and is valid. But in order to have auto-adjusting left-right margins ("centering"), you must also declare a width. I chose 90%, but you can make it whatever you want.