As Misty said, open includes/templates/classic/css/stylesheet.css
Find the line you mentioned:
#logo, .centerBoxContents, .specialsListBoxContents, .categoryListBoxContents, .centerBoxContentsAlsoPurch, .attribImg {
float: left;
}
Remove the #logo (and the comma):
.centerBoxContents, .specialsListBoxContents, .categoryListBoxContents, .centerBoxContentsAlsoPurch, .attribImg {
float: left;
}
Under that, create a new statement for logo:
#logo {
float:left;
display:none;
}
When you see a list of class and id names in a stylesheet statement separated by commas, it's just a way to make the stylesheet file smaller by combining elements which have identical style elements into one statement. You can always separate them if you want to style one differently, as above.