I'm going to show you a pretty easy way for skin cleaning with Photoshop. I'll try to do everything as simple as possible, but you should now at least the basics of the program, like how to select, and the management of layers.

First grab your image. I'll use this close-up of Scarlett Johansson taken from scarlett.host.sk:

Now we'll select the zone to clean. Just select Quick Mask Mode from the menu at the left, and draw a mask over the face in the picture. Is exactly the same as those natural masks old women wear at night or a the hairdresser, so leave eyes and mouth out.

In Quick Mask Mode, when you paint, you create an area that will be converted to a selection area on exiting this mode, so paint the mask all over her face.

When you finish masking, go back to the Normal Mode, so everything out of our mask is converted to a selection. Go to Select -> Invert, to get what you painted selected and copy it to the clipboard.

Create a new layer with the selection using Layer -> New -> Layer via copy or pressing CTRL+j so you get this on a new layer:

Use the Dust and Scratches filter to clean this layer, and it will be done. Enable the visibility of the two layers with the 'Eye' icon at the layers panel, and check the difference of the after and the before the technique. Mouth, eyes and nose holes are very important, because they will be blurred if we haven't done it. Play with the mask selection and the filter settings until you master the effect and get acceptable results.

If everything has been done right, you should be able to see the diference by just enabling and disabling the second layer.

My results (pass the mouse over to view the difference):