You set the margins in the CSS file for the <div> in question. If the div is named 'wrapper', for example ( <div id="wrapper"> ) you would enter this into the CSS:
#wrapper {
margin: 15px 0 0 15px;
}
remember the order when entering the values is Top, Right, Bottom, Left.
You can also just use 2 values if the top/bottom, and left/right are the same:
#wrapper {
margin: 15px 10px;
}
This will set the top & bottom to 15px and the left and right to 10px.
You can also set values for each position individually:
#wrapper {
margin-top: 15px;
}
Don't forget the ; at the end of the values. All CSS codes can be found here,
http://w3schools.com/css/default.asp.