Anyone who is looking to learn to paint in Photoshop is well-advised to examine Craig Mullins website IN DEPTH.
The address is http://www.goodbrush.com
The thing about painting is that it involves seeing the world two-dimensionally instead of the 3d forms that your brain translates too. The hardest part of painting is actually seeing. This comes easier to some than to others, but rest assured that a tutorial won't give you good results. Only long hours of practice drawing & painting from life will give you the proper mindset needed to paint realistically (in Photoshop or otherwise).
While learning to paint, you should be concerned primarily with form. The forms of the object and their hilights and shadows. The overall aesthetic (ie how blended it is, or nature of the line) is unimportant if you can't control the form. This is particularly evident in Craig Mullins' work when you look at some of his photoshop 'sketches'. He can create a scene in 100 brush strokes that looks almost photographic when zoomed out because the nature of the forms are captured so well. It's also interesting to note that while form has to do with shape and proportion, it is not a simple mathematical relationship. There is something intangible which makes something look the way it does, and the only way to understand that is by doing a lot of bad drawing and figuring out what's wrong with it. In the early stages I think a teacher is almost necessary to make fast progress, but the most important thing is dedication and to never be satisfied with what you've done.
-jiblet