Standard deviation calculations with excel

Hi,

Would someone be able to help me with standard deviation within excel? I have used the formula =STDEV(number one, number two...etc). I think the problem is that my data is not a normal distribution, thus the incorrect standard deviation value. But I need to some how calculate standard deviation? Is there a way that I can with excel? I have over 20 000 data elements, thus making it harder to work out. Could someone please suggest some ideas that I could possibly implement to help correctly calculate standard deviation?