Essentials

Here's the table which shows all about "how to paint". This small harmonogram is easy to remember and explain exactly what to do and when. Essential for everyone who like to draw, paint... 
START----FINISH
FAST----SLOW
LARGE SHAPES----SMALL SHAPES
LOOSE MARKS----TIGHT MARKS
LIGHT MARKS----DARK MARKS
What that all means :
At the beginning of painting start whit loose, large shapes. What that means is that is not a good idea to go into any details at the beginning. Good option is to compose some background whit loose texture. The first goal is to compose a raw sketch of background and a object which will be our main theme of painting. A creature for example. No details at all at beginning. When loose and raw compose is ready We can start whit some middle light tones to establish light source. At this stage we slow down a little bit to get good felling of light and its source. Its a very important step. After this is done a viewer should be informed enough what is going on at the painting. What the source of light is and about composition overall.
At this point we adding more and more details. Remember not to break light composition of Your painting. Important thing is to keep correct transitions on oval and sharp objects. At oval shapes like spheres light transitions have to be smooth. At Sharp object like triangles light needs to break suddenly. There can be also variations of this two where oval shape go int sharp object in some point. Photoshop offers a loads of tools to correct errors and mistakes so don't be afraid to experiment a lot. Its good idea to use a new layer for new idea for Your composition. If You will not be satisfied about new idea You can simply delete or correct what's on the layer easily whit out interrupting into the rest of composition. Liquify filter is good to correct the overall shape of an object, but do not overdose it. Overlay layer help a lot when it comes to adding shadow and light felling of Your composition. Overlay is also good to fill a piece whit colors. There is tones of tools. Try them as much You can. Experiment a lot whit them and Your painting. While painting switch Your image horizontally as much is needed. It will help You establish light source correctly and also symmetry of Your creature.
Remember that I've just start my painting studies about 5 months ago and I have never Study it at school. I'm a self-learner and I do not now everything so there is a lot more to write and tell than this what I brought here today. Be creative.
Good luck :]