Did you know that you DO NOT have to hit the AUTOSUM icon to add the numbers in the row or column.  Instead of clicking the autosum icon, just click ALT+= together.  That simple.

Also, I have seen some people who highlight the table with the headings and numbers and hit AUTOSUM and it will calculate the table for you.

First highlight the table.  I would click anywhere in the table then click CTRL+A to highlight the entire table! See below.

Then use the short cut, ALT+= to add the total row and total column.

Notice that it deselects the column headings and row labels and the totals are calculated for you.  This only works when you are adding numbers.