This topic has been beaten to death. Have you tried using Search?

One more time... There are several ways. Here's what I think is the easiest:

1. Make a duplicate of the track you wish to blur the face in.
2. Place it above your original track. Be sure it aligns properly.
3. Drag and drop cookie cutter FX filter on top track.
4. Use border setting in FX window to increase size so you can see default circle.
5. Use size in FX window to reduce size of circle to area to want to mask.
6. Apply the Gaussian filter for the amount of blur you want on top track.
7. Reduce or eliminate circle border, set color, etc. if you want.
8. Move the little gold square in the FX window to position the mask.
9. Resize, move the circle mask as needed setting keyframes as necessary.

The tricky part is watching the vid carefully if you want to keep the mask in place. To accomplish that be sure you click on the timeline, then advance through your video one frame at a time using the Ctrl key while clicking on the keyboard right/left arrow keys. Set a new keyframe adjusting the size and position of the mask as needed by nudging the little gold square in the FX control window. In this way you can get very precise control of the position and size of the mask, keeping it in place as your subject moves around.

That's the basics. See my tutorial over at Kelly's site. You can of course improve on this in several ways. A good next step is once you know how it works, make a image in Photoshop some similar application, make it a Alpha channel so it is transparent. Drop on track use FX pan and crop instead of cookie cutter. This way you can have a plane fly over a roadmap, cast a shadow, neat stuff like that.