Photoshop is the standard tool used in the photographic and graphic arts industry, and has been since 1991. Because of its market dominance an entire cottage industry has grown up around it. There are literally hundred of books about Photoshop, with tutorials, learning guides and the like. Most schools that teach photography and the graphic arts teach courses in Photoshop. There are dedicated magazines published each month just about Photoshop.
Most pros use it almost exclusively, making finding local assistance that much easier.
There are numerous web-based resources available, including this site. Of course Photoshop is the richest and most complex photo editing software out there. This means that you're unlikely ever to run out of capabilities. At least the program won't. While many of Photoshop's features are esoteric and will remain unused by most photographers, some like Channel Mixer, Duotones and the ability to work on 16 bit images make a huge difference in being able to achieve ones vision and not being hampered by the tools available.