gray image and segmentation...

I am rather new to matlab, but I was hoping someone could help with this question. So I have a color image that I want to convert to grayscale and then reduce the number of gray levels. So I read in the image and I used rgb2gray() to convert the image to grayscale. Now, i want to calculating the number of gray levels of it(min and max) with GLCM, after that how can i reduce the number of gray levels with: 1- histogram 2- linear transformation