If you have a list of values all in one column, you can use AutoSum (in the Formulas ribbon) to quickly obtain the sum.

AutoSum-ing numbers in vertical list

You can use AutoSum with a horizontal list of values too!

AutoSum-ing numbers in horizontal list

AutoSum is a powerful formula, however you need to make sure of one thing when using it...

Only use AutoSum if the cell containing the sum is directly in line with the cells you're summing.

If you attempt to AutoSum a list of values in a cell not in line with your values, AutoSum will not recognize what numbers you're trying to sum and won't work.