Adding a column (or row) of numbers is possibly the most common thing you first come across when learning about spreadsheet formulas. There are multiple ways to enter the SUM function.
Shortcut Key: ALT + = (That is holding down the ALT key, and the pressing the = key). Make sure you have selected the cell below the data to be added.
This works especially well if there are no gaps in the numbers you are adding up. Excel will automatically select all the numbers in the column (or row) and your sum formula is complete.
Another way to directly input the SUM function is to click AutoSum on the Home menu. There is no need to select all the data to be added. Just the cell that you want the sum to be in, (as long as it is directly below your data to be added) click AutoSum and then press enter. (You can highlight all the cells to be added and then click AutoSum, and then you do not even need to click Enter!)
AutoSum also has a little drop-down picker next to it, and you can access some other functions from here too.
You can also, just type “=sum(” and then select the cells to add. This is useful if the sum does not sit directly under, or to the right of the cells to be added, or there are gaps in the data. Both the above methods will not work in that instance.
Of course, you could also use a Table. This gives you options to add sums (and lots of other functions) into your total row, without having to type any formula. Just choose what you want from the drop-down menu in each total row cell.