Start by opening the photo you want to remove the wrinkles from. In this photo im using, I want to remove some of the wrinkles around the eye, and smooth out the cheek area.

For this tutorial, we will be working with a tool call the "Healing Brush" - it is located next the the BRUSH tool in your tools pallette, if its not visible, then you need to click on the small arrow to select the brush

Now, for the brush size, it will depend on how large the area you are working with, for this photo, about 15 - 20px brush is a good size. Set your brush setting to like this

NOTE: as a safety pre-caution, duplicate the original layer and perform all modification on the duplicated layer, if anything goes wrong, you will still have the original.

Now move your brush over to the image, to a "good" area of the skin (NOT THE WRINKLE AREA).
Then, hold down ALT, the cursor will turn into a crosshair, this indicates the sample area. now LEFT CLICK on the mouse to select that area as a sample.

Now, let go of the ALT key, then slowly brush over the wrinkled area NEAR that sample area,

What we've just done is taking the sample area and replacing the wrinkled area with the sample. Often you may have picked a bad sample area and making the image worse than original. If this happens, just undo and try again, selecting a better area for sampling.

By repeating the above technique, i've removed the wrinkles from this photo, as you can compare below.
Note: Be warned though, do not go overboard and remove all the wrinkles, it will remove all the characteristics of a person and probably make it look obvious and fake.