Any artist who depicts the human form must know the basics of human anatomy. Short of getting a medical degree, there are simpler ways to learn the basics. Many books on anatomy are specifically designed for artists and cover the proportions and basic shapes that make up the body. For example, a typical human is 8 heads tall (that is, the size of the head represents 1/8 of the total height). Reading one or more of these books and completing sketches of how the finished sculpture will look is a good first step toward an accurate figure sculpture.
In addition to sketches, photo reference will make the sculpting process much easier. If you have access to a live model, take photographs of the model in the pose you plan to sculpt from many different angles. If a model is not available, find photos or other artwork of a person in a similar pose. Combined with your new knowledge of anatomy this will ensure an accurate final product.
Choose a sculpting medium based on your knowledge of the materials and the type of sculpture you wish to produce. A subtractive method, such as carving in stone of plaster, is best for an upright or reclining pose that will allow the sculpture to support its own weight. For action poses, additive processes such as clay or casting in metal may be best. Look at the work of other artists to determine the style of your sculpture and to help in choosing a medium.
For many types of figurative sculptures, the best way to start is with a simple wire armature. An armature resembles a stick figure and can be copied directly from your sketches or photo reference. Once built, you can modify or pose your armature in a variety of ways until you determine the final pose. Later, the armature will help provide support for the sculpture by giving it a rigid metal skeleton.