The rules of your science fair may require you to give an oral presentation of your project. This is usually the hardest part of the project so you should thoroughly rehearse your presentation. A good way to practice for a perfect presentation is to record the rehearsal on a video camera. You can then decide for yourself whether you are speaking too slowly or too quickly, or whether you are making yourself clear.
In your oral presentation, you should go through each of the major sections of your project (namely, the problem, the hypothesis, the procedure for your experiment, the results and the conclusion).
You must be ready to answer all of the judges' questions related to your project. A well organized display will give the judges most of what they are looking for. However, the judges will often ask you questions simply for the purpose of finding out whether you understood what you were doing in the first place!
Be pleasant and enthusiastic when you're speaking to the judges. Don't be afraid if you can't answer all of their questions. Instead, you should offer information about other aspects of your project that might interest them.
Above all, you should have fun at the science fair. If you're enjoying yourself, very little can go wrong, as long as you have done your homework!