It really depends on where you are with graphic / design outside of painting. If you have a good working knowledge of software like Photoshop it can help you get started far more quickly; for example getting your head round layers and blending effects.
There are a few tutorials on the following sites:
Project Open Sky
...but one of the best ways to get started is to download a paintkit, open it in Photoshop (or equivalent) and see how the whole thing is put together by showing / hiding layers one at a time.
I need to develop my own site (Flightsim Paints
) a little more and add some tutorials...