Hi,
If you've not done any painting at all, then the place to start, I suppose, would be to get familiar with the paint program itself.... which can be quite daunting to a new comer because there is a lot of tools and features. To help with this, alpine wrote up a description of some of the most common tools in paint shop pro. You can find it here.
http://cownap.com/~mtmg/trucks/painting1.html
(oops, the link at the bottom doesn't work anymore)
Painting a truck, or even just blanking one out, is done by using the available tools to achieve the effrect you're after. There is no single way to do anything since most things can be accomplished a number of different ways. As you become more familiar with painting, you will find or develop the methods you like best. To help with this, you can search the net for painting tutorials.
http://www.google.com/search?q=paint+shop+pro+tutorial
And Cale has prepared a painting run through too, which is here:
http://cownap.com/~mtmg/trucks/psp7.htm
Hopefully, this should be enough to get you started.