You will need one of the Apple video adapters. If your MacBook Pro is the one in your profile (2.53 GHz), then you will need the Mini DisplayPort to VGA adapter. Unless your projector has a DVI connector, then you would need the mini DisplayPort to DVI adapter. There's also a dual-link DVI adapter, but no projector that I am aware of would need that one.
Look up the one you need, depending on the projector connection that you have- at the Apple online store.