The lasso tool is probably what you're looking for...

1. Zoom in to see the entire face

2. Select the "magnetic lasso" to isolate face from background

3a. Copy to capture face and paste elsewhere or
3b. Invert selection and delete to keep just face

I might use another technique.

1. Have Layer 1 be the image where you want to replace the face. Create a mask for it.

2. Have Layer 2 be the image where you want to use the face for the image in Layer 1.

3. Select the mask on Layer 1 and with the pen tool set to black, "remove" the face. As you remove the face from Layer 1, the face below from Layer 2 should start showing.

Layer masks are good because they're non-destructive. If you erase too much, you can set the pen to white and "add" the image back again. I learned about layer masks on this tribe; one of the most useful things ever!

also for the future you may want to check out the Extract option under Filter. If you are trying to lasso around hair it gets tedious and Extract is a good thing to check out.