The basic idea is to find a center position for your pie fan. Every triangle will have one of its verticies at this center position. Then, you want to be able to create a function which accepts a percentage value (0->100%) and figures out from there how many triangles to draw and where they should be placed. You may want to use an angle or radian value as an alternative (or overloaded member function). You'll also want to feed in the radius to indicate how large your pie should be. After you have all of this information, it's just a matter of using simple trigonometery to figure out where to place the other two verticies for your triangles.
It should look something like this:
Vert0 = new Vector2(center); Vert1.x = center.x + Math.Cos(theta) * Radius; Vert1.y = center.y + Math.Sin(theta) * Radius; Vert2.x = center.x + Math.Cos(theta + stepsize) * Radius; Vert2.y = center.y + Math.Sin(theta + stepsize) * Radius;