A volcano project for a science fair or class assignment is typically composed of three elements: a inverted cone made of clay or Paper Mache with a hole at the tip, a plastic tube inserted into the hole filled with dry baking soda at the (closed off) bottom and subsequently filled with red-colored vinegar to initiate the 'eruption', and a brief display explaining how a volcano normally works.
So why cover what is almost universally considered over-done like the volcano project? Because, occasionally, it is a required assignment, and because, occasionally, the person interested in doing this experiment is really interested in volcanoes and other geothermal phenomenon. There is also still some mileage in a well done volcano science fair project if the experimenter has a unique take on it.
In either case, the brief description above will probably be enough to get you started on the model and the remainder of this page will give you good information about the actual workings of volcanoes and links to more volcano information resources.
Volcanoes occur due to deeply buried (about 50-60 miles or more) magma (liquid rock) being pushed up to the surface of a planet (like earth). They often occur at the edges of tectonic plates, although hotspots (volcanoes not occurring at the edges of tectonic) plates do occur. When the magma reaches the surface it will erupt from underground (often violently.)
Once it reaches the surface it will cool to form mounds of solid rock. Over the years, an active volcano can erupt many times and for varying amounts of time. This action eventually creates a mountain of rock. During a particularly violent eruption, the top of this mountain may be blown off due to the gas pressure built up as the magma rose to the surface. This action creates the often recognized crater at the top of volcanoes.
If you need ideas for different ways to build your volcano project, Volcano World has over 10 different ways to make a volcano model - depending on your requirements, some can be done in a few minutes!
PBS has on-line resources about various interesting topics. Savage Earth is one such resource dedicated to volcano information.
Wikipedia has a lot of great informantion about volcanoes in general.