The hypothesis is your general statement of how you think the scientific phenomenon in question works. Your prediction lets you get specific -- how will you demonstrate that you hypothesis is true? The experiment that you will design is done to test the prediction.
An important thing to remember during this stage of the scientific method is that once you develop a hypothesis and a prediction, you shouldn't change it, even if the results of your experiment show that you were wrong. An incorrect prediction doesn't mean that you "failed." It just means that the experiment brought some new facts to light that maybe you hadn't thought about before. The judges at your science fair will not take points off simply because your results don't match up with your hypothesis.
Continuing our tomato plant example, a good prediction would be: Increasing the amount of sunlight tomato plants in my experiment receive will cause an increase in their size compared to identical plants that received the same care but less light.
