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You can work through the Image Processing Toolbox tutorials, otherwise, there is a good textbook:
Also look through Steve Eddins's blogs on the subject:
There are many online resources as well.
Of course, it's also helpful to look through the many, many thousands of questions that ImageAnalyst has answered on this and other forums!
A book by Rafael C. Gonzalez on Image processing using MATLAB. This should get you started.