C1 = A1 + B1
C2 = A2 + B2
C3 = A3 + B3
...you don't have to click on each cell of column C and add the formula repeatedly. Instead, you can:
1. Set formula for cell C1. And press the Enter button.
2. Select cell C1. You can see a little blue square at the bottom-right corner.
3. Move the mouse over the little blue square until it becomes a cross. Click down and hold the left button (or right button if you are left handed). Drag the little blue square all the way down to the last cell of column C.
4. Now you can see that all the cells of column C are correctly set as the sum of the corresponding cells in column A and B.