How to learn retouching? At first glance retouching may seem over-complicated and very hard to learn, especially if you are not familiar with Photoshop. But in fact, every person can achieve significant results in improving images. You may not become a professional retoucher, but "WOW! GREAT JOB!" from your friends is guaranteed.
Most image problems may be corrected using rather straightforward and clear techniques. A bit of analysis (which is the first and one of the most important steps in retouching), then 5-15 minutes for intensive Photoshop tools usage - and your image looks much-much better.
If you want to go further and do more professional image editing, the importance of analisys increases greatly. If you can clearly identify the problem and the "right" ways to correct it, you may achieve the result in several minutes instead of hours.
So the first thing in learning retouching is developing your ability to "see" what is wrong. Give a "problematic" image to 10 people. Some of them may not see a problem at all. Some may say "hmm... something wrong with this image", some may be more specific and say "bad contrast" or "unnantural colors".
You have to "see" what exactly has to be corrected, e.g. "there is a problem with white balance - image is too blue, shadows are not deep enogh, skin has a foreign color tint, blue channel contains much digital noise, sharpening is needed after resizing, eyes have to be accentuated" and so on. If can identify the problem correctly, the ways to correct it are usually obvious.
In general, to succeed in image retouching you have to learn three main Photoshop concepts: layers, curves and channels/color modes. If you add selection tools and unsharp mask filter , you will get nearly everything you need. And at last, actions may increase your productivity greatly.
All other tools (and there are dozens of useful ones in Photoshop) are rather easy to understand and usually students do not have any difficulties with them.
So what you need to succeed in retouching? Passion for learning and a lot of practice are the key to success. Not very original, but 100% true :).