Insert a Function in Excel Spreadsheets

You can get help putting together a function by pressing Shift + F3.
This gives you the Insert Function dialogue box where you can search for a function, choose the function and put in the details. Use Tab to move through the fields, Arrow keys and Enter to select.
As with Autosum, if you are in a cell next to a range of numbers, it will automatically detect the range and put it in the function for you.
If you are positioned on a cell with an existing function, Shift + F3 will let you edit the details.