http://se.mathworks.com/matlabcentral/answers/10716-how-to-normalise-image-intensity

See for example , if that is what you want: http://se.mathworks.com/help/images/histogram-equalization.html

Here is "A tutorial on intensity transforms of images"

http://matlabtricks.com/post-45/a-tutorial-on-intensity-transforms-of-images

it will probably help you understand basic concepts as well as their

respective matlab code. 

You can do it by two ways in an image I(m,n):

1) Divide every I(m,n) by 255.

2) Find maximum intensity value max of I(m,n) pixels. Now divide every I(m,n) pixel by max value.