Time for action — pressing A to select all
Pressing the A key to select all the vertices is the easiest selection to do. It selects all the vertices, or deselects them if they are all selected. It's often used to clear the current selection, so that you can choose a different set of vertices:
Click on the Vertex Select Mode button in the 3D View header button with theLMB, so that you can work with vertices.
Press the A key several times.
What just happened?
Pressing the A key either selects or deselects everything. You worked with vertices, but this applies to edges, faces, and objects as well. If you look just above the 3D View window on the right as you do it, Blender shows you how many vertices you have selected.
