Not an expert, but done public speaking a few times myself.
Body Language: That really depends on what your speach is about. If you are presenting something to be persuasive, try to keep a strong stance to make the listener feel the power behind your words. If you are talking about something of their interest, try to just be relaxed and take in the bearing of the crowd. Don't make anything too elaborate though: You want them focused on your words, not the crane style you are doing.
Facial Expressions: Again, depends on what you are presenting. However, if you have key points, try to emphasize those. Elevate your voice and change your tone when saying something something important, and your face will move naturally. However: Make sure that you aren't constantly frowning/etc. People tend to react more positively to the speach on someone who smiles.
Eye Contact: Try to simply gaze around the room, in a smooth manner. Most young people feel pressured when looked directly at for any period of time. Don't jump from face to face, pretend there is a bug flying around the room, and follow it. That way, most, if not all, of your listeners will feel that you were talking to them, but not focusing on them.
However, if you are presenting something different, such as a monologue from a play, try to focus your attention of as many spots/areas as there are people you are talking to, that way your listeners can take in the atmosphere of your speech.
Also: Don't be afraid of asking the organizer of the even if you may use a podium to stand behind or a chair to sit on, whatever will work for what you are presenting.
If you provide more information on what you are presenting, more accurate help could be given.