Having only done one myself (and doing badly- possibly due to the three days of diarrhoea beforehand possibly due to the inexperience possibly just due to me sucking.. ok all three) but I CAN tell you what most people WILL say:
-Sort out your cardio: you'll be pissed off if you gas and that's the only reason you lost
-Train your takedowns/standup grappling: You'll also be pissed off if you loose because you got taken down off the bat straight into side/mount etc and couldn't even work the guard.
-Have fun: If it's your first the general consensus is don't expect to win but expect to learn about competing in general and about holes in your game.
I, for example, learned that a surefire path to sucking is not to open my guard at all and just to sit there- I've since forced myself to be a little more offensive and it's genuinely improved me MASSIVELY (obviously). And that's just from one match... which I lost :redface:
Enjoy the atmosphere and the buzz you get for just competing.
Where you training, btw?- EDIT: Never mind- just saw you're a scouser (or at least living in L'pool).