How to work Photoshop and how to do portrait retouching, while related, are two very different things, and you generally need a good working knowledge of the first to get into the second.
I learned the first on Lynda.com watching instructor Deke McClellands first series of Photoshop One-on-One classes many years ago from the beginning classes to the mastery classes, and it really, REALLY worked great for me. I see he's updated the series with each new version of Photoshop since. Just like school, it starts off with the fundamentals and then uses them as a foundation to build upon, up and up and up until you've got a really good understanding of all the tools and techniques to use and how.
A lot of people recommend just watching Youtube videos, but to me, that was a shotgun approach that didn't provide a logical order to learn in. So many techniques require prior knowledge of tools and techniques to do them, that you need that prior knowledge first. With a real photoshop class, you get that training directions from beginning to end. With Youtube, you don't.
Once you get really familiar with the tools and techniques, you can learn how to use those tools and techniques to specifically do portrait retouching, and for that I recommend Phlearn.com and guru Aaron Nace.
