I would say use a black Solid layer in which you create a circle mask that you animate.
1) Go to Layer Menu and choose New > Solid > Black
2) Select the layer in the composition, and then select the Ellipse tool (left of the Pen Tool, where you see a rectangle)
3) Go to the first frame, where you want the circle big, and clic and drag the Ellipse tool in the view, it’ll create a circle, the size you want.
4)Uncollapse the Solid layer, and its Mask layer, and clic on the stopwatch for the Mask Path : that way, you create the first key of animation).
5)Now go to the frame where you want your circle animation to end (full black), and rescale the circle (double clic on circle) until it’s just a dot and your screen is completely black. (it automatically creates the new key)
Then, you may just have to shift the positions of the circle (just select it and shift it) or its scales (make sure the Current Time Indicator is on the keyframes).
That should be it! I hope it’ll help. 🙂
Eva