So what have you tried? If nothing, then you need to learn MATLAB. Start with the OnRamp tutorial.
Start by learning how to read a file. There are god knows how many ways to load in information from a file. It depends on the file what you might do, but many things can and will work. Why not read the help for tools like load, textread? But many other tools will do the job too.
Once you understand how to read in your data, then you need to work with it. How is the polygon defined? Are these just random points, and no order is given? If so, then you need to figure out the order. Is the polygon convex? Then a convex hull will be simplest. If the polygon can be some completely general one, and you don'y have a clue about the order of a scattered set of points, then you probably need to learn a tool like the CRUST algorithm. You can find one from the file exchange as I recall.
If the points are already stored in a specific sequence to define the polygon, then it is simple. You compute the distance between each paitr of points, then add up the lengths of each line segment. Again, this is simple, at least unless you have not a clue. And then you need to learn MATLAB.
It sounds like you need to start by learning MATLAB. We cannot teach you an entire course here from the basics though, and much of that is already written.
