The most fundamental unit of a mesh object is the vertex. So, now you are going to make a vertex, and when you make a second one, the two will be connected by an edge. Then, you continue until you have enough vertices for a face.
Load a new file.
Press 7 on the Numpad to get the Top View.
Press the Tab key to edit the cube.
Press A to select all the vertices. Press X to delete them.
Look in the lower left corner of the 3D View window. It still shows that the active object is the cube. Only the data block of the cube remains. It has no connections to any vertices, edges, or faces. You deleted them. But, to delete the cube itself, you would still need to press X while in Object. Don't do that.
Click the 3D Manipulator button in the 3D View header, so that the 3D Manipulator icon does not show up in the 3D View.
Hold down the Ctrl key, and click theLMB somewhere to the left and above the 3D Cursor to create a vertex.
Press Ctrl+LMB three more...