I started using Photoshop in 1999 as a freshman in college. As a photojournalism student at Western Kentucky University I took several classes dedicated to digital design, multi-media and photography. A large chunk of the first digital class was on the use of Photoshop, but as a photojournalism major we were limited in what we learned and, for good reason, what we were supposed to do a photograph.
I still believe that if you are reporting the news the photographs should not be manipulated. So if I were to tone a photograph for a newspaper I would limit my work to color correction, cropping, removing dust spots (I still remember removing film scratches!) and lightening and darkening the image while still being true to the event and maintaining realism.
That being said my advanced education in Photoshop didn't begin until 2013. Since starting this new higher education in photo post processing I am constantly amazed at the ever evolving capabilities of Photoshop. (Honestly, I'm slightly bitter that I spent 14 years thinking I knew something so integral to my photography and then realized I knew next to nothing.)
Over the last two years my knowledge of Photoshop has grown by leaps and bounds but there is still countless things for me to learn. There is just so much depth to Photoshop that it could easily take up two full courses in college. I feel my current limitations are mainly due to speed, consistency and my own imagination. I try to practice the skills I've learned so they are done smoothly and without requiring me to do it again and again. Photoshop is a tool that can be taught, but like photography, it requires practice so that the final results look flawless and un-manipulated.
There are multiple sources to learn Photoshop. Here is a small list of ones that I have used and like:
My favorite by far is www.phlearn.com. Phlearn has hundreds of free tutorials on YouTube, but if you do these you will have to use your own files to practice. If you really want to learn quickly buy one of their tutorials. They supply files and have dozens of tutorials to choose from. Aaron Nace and the guys at Phlearn have a great thing going and I would not be anywhere near where I am without them!
Next up is www.lynda.com. If you have not already heard of lynda.com it is a monthly subscription service and has several instructors to chose from. I really liked Chris Orwig. Very comprehensive and hours of videos, Orwig, soothes you with Photoshop knowledge. If you pay a little extra you can get the files he is working on but personally I used my own files. Also, I believe lynda.com offers the first month free, but once you start down the rabbit hole you will be happy to pay.
Next is www.creativelive.com. They have an interesting model. They stream tutorial workshops live 24 hours a day. If you catch the program live or in rerun its free. Let me reiterate, countless hours of tutorials from some of the best Photoshop retouchers, photographer and business people in the business... 24 hours a day... free! They have one week dedicated to Photoshop every year with dozens of talented retouchers and photographers teaching. You can buy any of the videos after the original broadcast and considering the amount of material covered it is well worth the money. My favorites tutorials for Photoshop are from Pratik Naik and Aaron Nace, but there are several others.
Another great resource is www.retouchingacademy.com. They have several video tutorials available and now a digital magazine for the iPad. I have bought the hard copy the book Digital Photo Retouching: Beauty, Fashion & Portrait Photography by Julia Kuzmenko McKim which has links to tutorial videos.
Lastly I'd like to give a shout out to www.RGGEDU.com. Although they don't have a tutorial dedicated to Photoshop they have Photoshop in every one of their photography videos. Most recognizably Michael Woloszynowicz and Erik Almås. Both are fantastic photographers but achieve amazing results by combining both skills to make beautiful art.
Although all of these cost money all of them also have some free material either on their blog, YouTube channel or clips from the tutorials. Check them out and have fun learning!