First, you need to install it in your quick access toolbar. So follow the steps:
Right mouse click on the Autosum button (Σ button under Home tab) > select Add to Quick Access Toolbar

The Autosum ( Σ ) tool is now added to the tiny toolbar above the Home tab.

Now press Alt on your keybord > it shows you the number of your new shortcut key (here it is 4) > so your Autosum shortcut combination is Alt+4.

By the way, if you press Alt again, the numbers will disappear.
So the shortcut key is installed.
Now try a calculation:
In Column A, I have the numbers to sum up. I want the calculation in cell B4.
Select cell B4 (or any cell where you wan to do the sum) > press Alt+4. It will show you =SUM()

Select your range (in this case A1:A3)

Press Enter. You have the sum.

If for any reason, you would like to remove the Ausum ahortcut from the tiny toolbar, where you just added it, then right mouse click on it > select Remove from Quick Access Toolbar. It will be removed immediately.

Do these practises even for fun, it makes you an Excel Master

By the way, I am using Excel 2007.
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