Select a column:

A

Use Shift+Click to select another column:

Hold down Shift, click on B

Or you can use Ctrl+Click to select columns that aren't side by side:

Hold down Ctrl, click on D

You can do the same for rows.

To select the Whole Sheet, click in the top corner:

You can also use the keyboard to select cells:

Arrow keys
Shift+arrow keys
Shift+spacebar (selects rows)
Ctrl+spacebar (selects columns)