Although shape can be effective as a two-dimensional description of appearance, form adds the third dimension; depth and reality. The appearance of form depends on the way light strikes an object, the transition from highlight to shadow that produces roundness.
LIGHT
You can use either natural daylight or studio lighting to highlight form. One common mistake many people make is to over-light subjects, which kills forms, so keep lighting simple and not multi-directional. A strong, overhead light such as that produced by the sun at noon on a cloudless day is undesirable. That kind of light makes an object look flat and uninteresting, or worse, lost in black shadows and blown-out highlights. The type of light that brings out the form of an object is called "golden hour" light, the kind that only exists in the early morning or late afternoon. Magic hour light is softer and warmer, and the shadows are stronger.
Choosing the correct light is almost as important as choosing the correct angle. Light that comes directly from in front of an object will make it look two-dimensional. Light coming from behind will make it a silhouette. But light coming from either side creates subtle shadows, which will expose the object's contours.
ANGLE
You can play with perspective in order to change an object's form - at least in the eye of your viewer. Shooting from below or at ground level can make an object appear larger than it actually is. Shooting that object from above will make it look smaller. Angle is a good way to communicate an object's importance, and though it may have little impact on the actual outline or shape of that object, it can have a profound effect on how the viewer sees the object in terms of form and the way that object exists in three dimensions. Angle is also important for the reasons we discussed above; changing the angle can change the way the light falls on the object, which will in turn make it seem more or less three dimensional.
DEPTH
Form can also be highlighted through your subject's relationship with other objects. Placing objects on different planes, or different points in the scene from foreground to background, will reinforce your subject's existence in three dimensions. For example, placing one in-focus flower in the foreground while flowers in the background fall out of focus can help create depth--and depth enhances form. You can achieve a similar effect by placing your subject in the background and letting the objects in the foreground fall out of focus. This is called "selective focus," and it can be a powerful tool to bring volume and depth to your photographs.
PROJECT REQUIREMENTS
Now take what you've learned with you into the field. Experiment with times of day - chances are, photos taken in the morning or late afternoon are going to be better than those taken at mid-day, but having one from each time to use as a comparison will help you get a better idea of what you should be avoiding and what you should be striving for. When shooting your images, think about lighting, angle and depth.
Shoot a variety of images, you will be turning in your top 3.