no clue on the structure of your page #'s ... a link would be helpful :-)
basically what Darran said is true. there are a couple ways to go about it, and text-align plays a role in some....
the manner I use most the time is by limiting the width of a division, such as #wrapper in classic theme... if you define a width and then define margin on the sides it usually works just fine... example follows:
#wrapper
{ width: 90%; margin-left:auto; margin-right:auto;}
or, you can simply do this:
body */add this to it or replace if previously defined
{width: 90%; margin-left:auto; margin-right:auto;}