Select a cell: Click it.
Select a range of cells: Click a cell, then drag a white dot any direction to encompass the range of cells.
If you drag the yellow dot, you copy the cell contents to the rows you drag over. If you do this accidentally, drag the yellow dot back to its original position, or press Command-Z on the keyboard to undo the action.
Edit a cell’s contents: Double-click the cell, then type.