The simplest selection involves a single cell. Simply clicking with the left hand mouse button while the mouse cursor is over a cell will cause that cell to become selected.
The selected cell is indicated by a dark double line with a small square in the bottom right corner. The selected cell is the the one that is currently in focus and will take any input for the keyboard.
To make a cell become the selected cell, simply move the white-cross cursor over the cell and press the left mouse button.