There are three ways to select a section of a form in Design view.
You could click the section selector, the box in the vertical ruler to the left of the section name. (The rulers help you keep track of a form's size as you work on it. A form should be not too big, not too small, just right for the users.)
You could click in the section bar, the rectangle with the section name in it at the top of the section.
You could click anywhere in the background of the section. (Anywhere not on a control, that is.)
When a section is selected by any of these methods, the section bar will be highlighted.