In oder to find out what the standard deviation of a data set is, I first must know the arithmetic mean of the data set. Arithmetic mean is just a more formal way of referring to an average, when you really get into statistics you will find that there is more than one mean, so it is important to know which mean we are talking about.
So yes I just need some kind of method to add up all the numbers in a data set, and then divide by the number of numbers in the set to get a mean.
Once I have my mean method I can use that in my standard deviation method. The process involves adding up a sum, the values of which are the square of the difference when subtracting a number in the set by the mean. Once I have this sum I just need to divide the sum by the number of numbers in the set minus one, and get the square root of that result.
Did you get all that? Well in ether case this seems to work okay for me: