To select a part, click on it with mouse button 1.
When you select a part, selection handles and a selection border appear on the part. Selection handles are small solid boxes that appear on the corners and edges of the part to assist you in manipulating that part. A selection border is a frame drawn around the outer visual bounds of the part.
When you select a part, if other parts are selected, they will be deselected automatically. This is referred to as single selection. The name of the part currently selected is displayed in the information area at the bottom of the Composition Editor.