You could just put a formula in a cell to calculate whatever you would like.
for example, if in column C row 3 you type =A3+B3 you will add the contents of row 3 column A to row 3 column B and place the sum in row 3 column C.
There is a formula button at the top of the screen for more advanced stuff.
Also, you can grab the corner of the cell you just typed the formula into and drag it down the column to copy the formula into all the cells below it. The row and column are automatically adjusted.