Ok then, but we would need to start from the very beginning
It seems such a black art that I do not even know what questions to ask

. I will look for a beginners guide first
It's not a black art, but there are many different ways of achieving the end results, and different extremes to take it to, it just takes a bit of time and practice.
There are a few different threads on here as tutorials ( Tiler65 has one down in the Tutorial section right at the top)
http://www.talkphotography.co.uk/forums/showthread.php?t=142782
There are also lots of tools & plugins to do it quicker, but sometimes they're a sledgehammer to crack a nut.
First question you have to ask yourself is, what's wrong with the image?
The original you posted looks fine to me, she's got good skin, very few lines, white teeth (they're in shadow, lighten them more it'll look odd) and hair in great condition. Why retouch it? At most, remove the hair that runs across her nose, but that's about it.
With retouching you could start with something like this 5 step workflow, each in their own layer.
1. Cleaning (clone out distractions, dirt, dust spots etc)
2. Healing (spots, wrinkles, smoothing)
3. Dodge / Burn (which is a black art to me...but whitens teeth)
4. Liquify ( tool of the Gods, but open to abuse)
5. Colour correction
You really need to consider what you're looking to achieve at the end. There's a post from Natjag in Nudes & Glamour that shows a shot with & without skin smoothing. The one with processing looks quite extreme, but is the one that will sell for magazines etc. The same level of processing may not be appropriate for a family portrait and I find varying levels of desire for processing from those people I shoot.