need help writing a program using for loops

Here's the question, write a computer program that can be used to determine the agnitude and direction of the resultant of n coplaner ofrces applied at a point. What I have now is that Fn.x=Fn cos(theta n) and Fn.y=Fn sin(theta n). I understand the concept of what I need to do, I just don't know how to do it. I want to understand it so if anyone answers please explain whats going on if you can. I want to learn, not just get it done. thanks.