Well, I'm self taught - been using Photoshop since version 5 (I think). Basically, I just read lots of books and tried out lots of tutorials - and gathered useful snippets of information from forums like this one, of course! Then it's just a case of practice, practice, practice. And then more practice.
Many night schools and adult education centres run courses that cover the basics of Photoshop if you prefer a more tutor/student-based approach.
If you want formal qualifications, there is the official Adobe Certified program (similar to Microsoft Certification):
Adobe Certified
I don't know how much the AC courses and exams cost, but I'm guessing they're far from cheap so you may only want to consider them if you are aiming for a career in professional graphic design.