Autocenter main container with css div - css

Question

I am trying to have the main container of my site to center automatically when the explorer window is bigger, or to stick to the left whenever the explorer window is narrowed.
How would I achieve that with CSS?
Most sites do it nowadays, bloomingdales.com for example.
When you make your browser smaller, the main container stays on the left, and as soon as you open your browser on the right side, the main container automatically gets centered.
Thanks!
My site: www.tahara.es
ADDING CSS:
body {
margin: 0 auto;
font-size: 11px;
font-family: Arial,Helvetica,Verdana,Sans-serif;
color: black;
font-weight: normal;
}
.mainContainer
{
position: absolute;
width: 850px;
min-width: 850px;
margin: 0 auto;
border: 0;
}

Answer 1

body {
font-size: 11px;
font-family: Arial,Helvetica,Verdana,Sans-serif;
color: black;
font-weight: normal;
}
.mainContainer
{
width: 850px;
min-width: 850px;
margin: 0 auto;
border: 0;
}

Answer 2

This should do it:
body {
font-size: 11px;
font-family: Arial,Helvetica,Verdana,Sans-serif;
color: black;
font-weight: normal;
}
.mainContainer {
width: 850px;
min-width: 850px;
margin: 0 auto;
border: 0;
}
View on JSFiddle
The key is that there is no position: absolute; on your div. That property removes the element from the normal document flow, such that it has no parent element. margin: 0 auto; determines the left and right margins from the parent element. Putting two and two together, you can see that they're incompatible.