To select an area, you have two methods: use a wand or use commands.
Using a Wand
//wand
Typing this command will cause a wood axe to appear in your inventory.
While holding the wood axe, left-click on a block to select your first corner or a cuboid area. Right-click on another block to select the opposite corner.
You can also highlight the selection by typing //drawsel.
Using Commands
Stand at one corner and type //pos1. Move to the opposite corner and type //pos2. You can do this on spectator or creative mode to make a selection in air or water, where the wand is less useful.