Cannot centre my theme

Can any one help!!

I have developed my own theme comprising the following:

footer.php
functions.php (for dynamic sidebar)
header.php
index.php
sidebar.php
footer.php
style.css

Whe I originally coded the page in Dreamweaver (from my Photoshop visual & before any theme development work i.e. cutting and pasting code from the default WordPress theme), I had a nice fixed-width, centred layout-using a wrapper div around the page content to achieve the centred layout. Now that I have cut and pasted my original page elements over different pages (as above list,and combined the php functions etc and created my theme, it just aligns left in the browser window.

How can I get back my centred layout?

As my page elements are now on several php pages I can’t use a wrapper div? or rather, I tried putting a wrapper div round elements through the different php pages, but to no effect.

Any help would be very much appreciated, I am a relative novice with CSS and PHP, so a simple explanation would be much appreciated.

Thank you.