The first step is to download the ImageJ software from http://rsbweb.nih.gov/ij/download.html
(if you haven't already).
Once you have the program, go to the File menu and choose Open... This will allow you to select the picture from its location on your computer.
When you have the picture open, you can then do a number of things to analyze it. Assuming you have a picture of the hair shaft in cross section (cut through), you can use the elliptical tool to outline the shape of the hair. In my version of ImageJ, this tool is the second button from the left, marked by an oval shape. When you click the button, you can draw an oval that best outlines your hair shaft. Once you have drawn an ellipse that best outlines your hair shaft, go to the Analyze menu and select Measure. A new window will appear, called Results. In this menu, you should see a number of measurements: Area, Mean, Min, Max, Perim., Major, Minor, Angle, and Circ. I will define each of these measurements for you:
Area is the area of the ellipse.
Mean/Min/Max - I believe that these numbers refer to the color values assigned by the program, and would not be relevant for your study.
Perim. is the perimeter of the ellipse.
Major and Minor are the lengths of the major and minor axes of your ellipse. The major axis is the straight line drawn across the widest part of the ellipse, while the minor axis is the straight line drawn across the smallest part of the ellipse, perpendicular to the major axis.
Circ. is a measure of the circularity of the ellipse. This value varies between zero and one. A circularity of 1.0 means that the shape is perfectly circular. The smaller the circularity value, the more elongated your shape.
My advice would be to record the relevant values (Area, Perimeter, and Circularity) in a notebook or spreadsheet as you go.
I hope this helps you get started. If you have other types of measurements you want to take, or other views of the hair, please describe them and I will do my best to help you get the measurements you want. Let me know if you have questions about what I've told you here.