From what I have seen, 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, you should be able 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 guidlines 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).
Erase some of the unnecessary guidelines. Draw the outline of the eyes, (which should be thin and triangular), as well as the outline of the ears and neck.
Sketch the outline of his hair. The hair should be very large and rounded, more so if his head is tilted forward. Add more detail to his eyes. When drawing his eyebrows, make sure they rest directly above his eyes. Draw the nose and mouth next, making them very small and close together.
Next, erase all the guidelines. Add the detail to his hair; with Dragonball Z characters, the more detail, the better, so don't be stingy. Make the lines on his hair as smooth and rounded as you can; make them follow the flow of the hair, rather than drawing them straight down. Draw the shading lines under his eyes and mouth, and add detail to his ears. Draw his clothing, his sword, and the muscles around the neck. Erase any unwanted lines and clean up your sketch.
Take your final sketch and color or shade it however you like. Personally, I like using Adobe Photoshop, but you can color with pencils, colored pencils, or anything else you want.
Next, we'll draw the 3/4 view. You can probably use these proportions 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 guidlines 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. Add the neck, too.
Erase all the guidelines. Draw the basic shape of his hair, making it very thick and rounded. Add his pupils, and shade beneath his mouth.
Draw the details of his hair, including the shorter wisps that hang down over his eyes and stick out from his head. Add the wrinkles above and below his eyes. Draw his coat, and add the ponytail (which is barely visible over his shoulder). Erase all unnecessary lines and clean up your sketch.
Now that you have drawn his face, you can shade and color it however you like. Yes, I realize I left out that little badge thing that goes on the shoulder of his jacket; I forgot to draw it. ^_^ Its just a circle, though, so I don't think you'll have too much difficulty adding it if you really want to.
Trunks is one of the characters on DBZ that can transform into a Super Saiyajin (yes, that is an accepted spelling, so don't send me gripy emails about it). If you would rather draw him in this form, there are just a few extra steps you have to take. First off, make his forehead a little bigger, and his facial features more slanted and grouped together. The lines of his face will be much more sharp and slanted, and his eyes will be much more narrow. For the hair, there are two different styles (well, that I found, anyway). ^_^ In the first style, part of his hair is pulled back; in the second, his hair isn't pulled back. When drawing his hair in the Super Saiyajin form, 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 these two pictures, 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 its spiky, make the hair full and rounded, rather than just using straight spikes.
Here's what the hair will look like when the guideline for the back of the head has been removed and the hair has been shaded. It's quite different from his normal form, which is why I felt I should go over it, at least a little bit.