Ahhh now this dpends very much on the layer (Div) you are trying to centre. If it's just one Div then
.content {width: 100%; margin-left: auto; margin-right: auto;}
in your css and
<div class="content">put your content here</div>
in the HTML will work but if you have several divs then it depends how they're laid out.
Have you had a look at the tutorials in
www.w3schools.org ?