These are very general questions, and it would be easier to give an answer to a more specific problem.
For your first question, the introductory MATLAB documentation goes through all the steps you need to create an M-File. Type "helpwin" at the prompt, then look at the "Getting Started" section, accessible by clicking in the Contents tab.
For image processing, do you have the Image Processing Toolbox? If you do, start by looking at the examples and documentation that come with it. There are lots of image processing M-Files in the demonstrations. There are also many image processing M-Files in the file exchange, but it is best to start with the examples in the MathWorks documentation, as they are more consistently good quality.
Finally, if you are doing image processing with MATLAB, it's a good idea to have a copy of this book to hand.
