Easiest way to use the Path object:

var myPath = new Path();

myPath.moveTo(100,100); //Starting point (x,y)
myPath.lineTo(200,100); // Draw a line to a new point
myPath.lineTo(200,200); //Continue to draw straight lines
myPath.lineTo(100,200);
myPath.lineTo(100,100);

Or perhaps:

var list = [Point(400,400),Point(400,500),Point(500,500),Point(500,400),Point(400,400)];
var newPath = new Path(list);

Try this out and then check out the reference section for the Path object at this site.

Not using the Path object but createRectangle() function instead:

var rect = new Rectangle(0, 0, 100, 100);
activeDocument.createRectangle(rect);