Shooting my cousin's graduation tomorrow..tips?

Well, my cousin's graduation is tomorrow and I really want to get atleast 3 or 4 good shots. I know her parents don't expect something great out of me, but it's an upgrade over P&S.

Had a 580ex come in today, but no one was at the house so I can't use it (anyone know where fedex does with the package and where I would find the location?)

I have a chance to get the flash by tomorrow when they open, but I don't know where FedEx put it.

So now I only have my XTi, 70-200 f/4, 50 1.8, and a 4.5 Sigma FE.

I assume the only the stage will be lit.

What should I capture?

If you can't get too close your 70-200 should probably be the lens of choice with your 580ex if you can get your hands on it. Even with the stage lighting you'll find that there won't be enough light for good shutter speed so you'll need to push your iso up don't be afraid of going up tpo iso800 if you need to and as long as you get your exposure right the noise won't be too much of an issue.

If you can get close then your 50mm will be your better low-light choice. A wide aperture will allow you to get a better shutter speed.