Most Dragonball Z characters can be drawn using these basic shapes and proportions. Dragonball Z characters all have similarly constructed faces: they have large foreheads, slanted, triangular eyes, and small lower faces. Once you see how the basic face is proportioned, it should be easier to draw whichever character you like. Begin by drawing a large, slightly elongated circle for the forehead. Draw the lower half of the face and divide it up with lightly drawn guidelines as shown. Notice that the lower half of his face can be divided up into equal sections; the main horizontal guidelines are equidistant from each other. Draw the slanted guidelines for the eyes, and sketch the position of the mouth (which should be directly below the guideline for the nose). Make sure all the guidelines are drawn lightly, because you are going to erase them later on.
Erase some of the unnecessary guidelines. Draw the outline of the eyes, which are just blocky, angled trapezoids. Make sure the bottom of the eyes line up with the slanted guideline you drew in earlier.
Next, sketch the outline of his hair. The hair is very large and spiky; notice how it smooths out slightly on the right side (our right, not his) of his head, though. Add more detail to his eyes and ears. When drawing his eyebrows, make sure that they rest directly above his eyes. Draw the nose and mouth next, making them very small and close together. The nose should be sort of like a blocky 'L' or wedge.
Next, erase all the guidelines. Add the shading under his eyes and mouth, and add detail to his ears. Next, draw the neck, muscles, and clothing. DBZ necks are usually pretty wide, so make sure the neck starts just beneath the ears. Erase any unwanted lines and clean up your sketch.
Take your final sketch and color or shade it however you like. Notice that the shading on his hair is very subtle, and gives the otherwise flat spikes more dimension, so you know his hair doesn't just go straight back like Ryoko's (from Tenchi Muyo), it kind of sticks out in all directions. ^_^
Next, we'll draw the 3/4 view. These proportions will work for other DBZ characters, too. Draw a large circle, then add the lower half of the face and divide the shapes up with guidelines.
These are pretty much the same shapes as in the front view, except they have been rotated downwards and to the side. The 3/4 view has less guidelines than the front view, but that's only because adding them would be unneccesary at this angle. We'll only be using the guidelines for the eyes, nose, and the central guideline that runs from the forehead to the chin.
Next, use the upper horizontal guideline to draw in the eyes and eyebrows. Draw the nose and mouth, making sure to draw the mouth very close to the nose. The features of the face should have very sharp angles. Notice that while on the front view his nose looks small, on the 3/4 view it is much longer and pointed.
Erase all the guidelines. Draw the basic shape of his hair, making it very thick and rounded. Notice again that the hair flattens and rounds out on the right side of his head. Add his pupils, draw in the eyebrow ridges above his eyes, and shade beneath his mouth.
Erase all unnecessary lines. Add the detail in his ears, and add shading to his eyes. I added a few light glares to his pupils out of habit, even though DBZ characters almost never have such details on the eyes. You can leave them out if you like, but I think they look fine. Again, his neck muscles are very large, so start them just below the ear. Add the rest of his muscles and clothes, then clean up your sketch and prepare it for the final draft.
Now that you have drawn his face, you can shade and color it however you like. Notice again that the subtle shading on the hair shows that it sticks out in various directions rather than just going straight back. Make sure to try to include this if you shade your picture.
Goku, like several other characters on DBZ, can transform into a Super Saiyajin (yes, that is an accepted spelling, so stop bugging me about it). From what I could discern from the internet, he has at least three or four different forms, but I really don't have the patience to go through each of them. The differences are slight, anyway, and if you know which features to exaggerate, its not that much of a problem.
To draw Goku (or anyone, for that matter) in Super Saiyajin form, you'll have to make a few adjustments to his face, as well as his hair. His head and body should be wider, while the face itself is smaller. The features are more slanted and grouped closer together. The lines of his face will be much more sharp and slanted, and his eyes will be much more narrow. The chest and arm muscles are increasingly larger and rounder for every further transformation. These adjustments can be used on any Super Saiyajin.
For the hair, basically just draw a series of sharply angled, pointed spikes that extrude from his scalp. It helps to draw the outline of his head, as shown in this picture, so you can tell where the hair should go. It's easy to make it too big or too small if you don't know where to position it. Even though it's spiky, make the hair full and rounded, rather than just using straight spikes. All the spikes should curve inward, not just stick straight.
Here's what the hair should look like when the guideline for the back of the head has been removed and the hair has been shaded. I did a sloppy job of coloring the hair, though; the shading should be much less rounded than it is in this particular picture. You can look at other pictures of him on the internet or elsewhere for further reference.