For creating Rectangular/ Square features, the default shape is a rectangle and holding down the SHIFT key will force the dragged shape to be square. Hold down the 'T' key when left clicking to start the shape, the start point will be treated as the top left corner of the feature rather than the center of the shape feature.
Left click to start the rectangle, then left-click again at the end of the first side, then left click again when the rectangle shape is completed to create it. Once the shape is finished, release the left mouse button and the Modify Feature Info dialog will appear allowing you to setup the label, classification, drawing style, and attribution for the new feature. See Editing Feature Attributes and Drawing Styles for more details.
Once the shape is completed, the Digitizer Tool will remain in the selected shape creation mode. Left click to start drawing a new shape, or right-click to choose a different mode.