How to make my site on the center of the screen

[eluser]cinewbie81[/eluser]
Hi guys,

I have a very easy website up to the live sites yesterday morning.

I received some comments this morning suggest that I should've change the width of the screen so the site is a set width, rather than taking up the entire screen ( i assume they are very rich and having a bid screen monitor)..

Now i've already fix its width to 900px like this:

Code:

#webcomtain {
    width:980px;
    background-color: white;
    float:left;
}

Now the page is appear on the left of the screen.. how can i set it to be on center of the screen? Anyone ?