who are you going with? is it an organised event?
go into it with an open mind but please be aware of what natural things can cause panic, "orbs" and sounds etc.
have you got a video camera with IR capability?
take a notebook, a torch (do not use unless you really need to)
Even if you find what to you is absolute proof, not all will believe it so best to look at everything with a mind that it may not be what you hope.
Oh and prepare for a lot of sitting still, in the dark, bored :)