If you are using Excel, you should use Average and Stdev to get the parameters and Normdist to draw it. For example, you can read
http://excelgraphs.blogspot.com/2013/03/historgrams-and-over... (I don't like how they make the final graphs, but the article explain how to use the functions.) (Be careful, because it's easy to use the wrong x axis, and draw the Gaussian curve shifted 1/2 histogram bar.)
If you like to understand the calculations and use less magical function, you can read http://www.phys.utk.edu/labs/plstatan.pdf . (If you want to read directly the code, go to page 5 to skip the theoretical background.)