Take the histogram
totalNumberOfPixelsInIMage = numel(grayImage)
[pixelCounts, grayLevels] = imhist(grayImage);
If you're a beginner, you must see my Image Segmentation Tutorial in my File Exchange: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862 because it goes over all the basic steps you'll need for a typical thresholding-based image segmentation, analysis, and classification.

