Margin around arbitrary widget

Hi! I’m using the Twenty Fourteen theme and spent the morning figuring out how to add a header widget. I finally did so, but there is a lot of dead space below it. How do I modify that? I’ve searched through the support topics for a few hours today and can’t seem to figure it out. It seems that one thing I need to do is start a child theme (and transfer my current changes over to that), but I didn’t understand how to do that, either. My page is . I’m really novice at the whole CSS thing, so simple terms would be the best. Thanks!