On thing i feel is essential is learning to use layers in photoshop, some places will allow you to download a PSD or photoshop document to see all the layers made up from an image they create. To me, I use layers constantly in everything and once you get the idea of layers, you'll find it pretty quick. I'd upload one to use of mine but they are HUGE to say the least, I mark all my layers as to what they are and you can turn them off and see what layer is responsible for what part of the image it's making.
I see you have gotten PS7 which is good but not compatable with many of the new and free shapes, images, patterns, brushes, styles and gradients, all are part of photoshop to make neat stuff.
Places like deviantart do have members that will allow you to download a PSD file of their work which may be "Merged or complete" but it gives you an idea of how they put it together.
I prefer at least PS8 CS2 since it has more compatabilities with newer stuff.
I still feel that it's a personal preference of how someone makes their art and the steps they take but good tutorials on the net are found easily and help alot, from beginner to advanced to out of this world.
Unlike others I don't have a grasp on the pen tool or paths, I use the polygonal lasso tool and ut images freehand, since I have access to extremely large images which helps alot, with just that tool, you can cut out a car and simply paint over it with the standard brushes, oryou can use the image and make it posterized, this is the easiest way for some starters.
Guys that do know the pen tool have a great advantage over ones that don't, it's cleaner cuts IMO and can be saved as a path to later outline the image. Once you get the hang of creating layers and adding what is needed and where they go, it gets pretty easy at least for me after about a month at it, others have been doing it for years and can create images in minutes that are works of art no matter how you look at it.
To draw a straight line in photoshop, I use the shape tool and set the weight to lets say 4 pixels. Open a new document, lets say 1024X768 which is standard wallpapers for most and a decent size image to start.
now start a line with the shape tool as a line, you will see at the top in shapes it looks like a slash next to boxes and some other stuff. Hold down the "shift key" and just drag it across however far you would want it to go, it's perfectly straight line now on a new layer as you now see in your pallette.
Now switch to the elipse tool and draw a circle using the same method, hold down "Shift key" and drag across as big as you want the circle, now yo have another layer for say a tire or wheels that is a perfect circle, you can duplicate the layer on the right layer pallete and simply move it to where you want it. now you have two tires and the bottom of the frame of a car. This is basic shapes if it helps.