[Sisom]

First of all, edit altskin.css, line 68

#header .line0 {
direction:ltr;
height:150px;
position:relative;


}

Change the height to something smaller, like 20px;

Then you need to change line 31
#content-container {
margin-top:150px;
padding-bottom:50px;


}

Change the 'margin-top' value to 20px, or whatever you made the height of
header .line0

I think that's all, I can't remember, but I had to do this myself last week for the first time.
But as am2003 says, use the Firebug add on in Firefox and it will really help to see what CSS you need to edit.