Photoshop paste into a layer mask

You want to create a layer mask and paste an image into it

1. copy the image you want in the mask
2. create a new layer and put black over the whole thing
3. add a layer mask and option click it to get into the mask edit mode
4. cmd + optn + shft + v to paste the image into the mask
5. cmd + i to invert it
6. profit

Look here