What is a mesh?

Exercise

1. Load the factory settings using File → Load Factory Settings. (default selected the cube)
2. Make sure the 3D View window is active.
3. Press  CTRL + SPACE  to toggle the manipulator on or off.

Edit Mode

• Press  TAB  once to go into Edit Mode on the cube. (press TAB TAB to toggle on /off edit mode)
• Edit mode has three modes. Default in vertices selected mode.Because you load factory settings.
  
• Delete all cube's vertices. Press X key then it will pop up a delete menu. Select the vertex in delete menu.
• Press ctrl+LMB to create a vertex then do this action one more time to create another vertex. Blender will auto join two vertices with a edge.

Create a face

1. Press A to de-select all vertices. If nothing is selected , press A key to select all vertices.
2. RMB click to select a vertex.
3. Shift+RMB to select multiple vertices.
4. Press F key to create a face.
5. Press Ctrl+Z to undo.

Move one or more verties

1. Using RMB or shift+RMB to select vertex.
2. Press G to active "grab tool". Now you can move mouse to drag the vertex.
3. LMB or Enter to confirm the change. RMB or Esc to cancel the changes.

Try it

1. Change the viewpoint, perhaps to one of the perfect views.
2. Add vertices and/or move the existing ones around in the new view plane.
3. When you're done, rotate to a new viewpoint to examine your work.