Fist, well done, hopefully it'll be the first of many. The main thing that I'd say is to say to your target companions that it's your first shoot and they will help you out. Other tips, learn the distances that the targets can be. For blue pegs, unmarked round the 20cm (Bunny) will be between 5 and 10 meters away. You shoot one arrow into each of the 3 faces of your column in any order, so if you shoot the middle one first then if you misjudged the distance you may hit a different face and score something. You can then adjust your aim for the other two faces. The 40cm faces are all between 10 and 20 meters, with the 60 cms between 15 and 30 meters, and finally the 80cm is between 30 and 45 meters. On the marked round they can go 5 meters longer, so try to get aiming marks between 5 and 50 meters.
If it's a two day shoot, you tend to be in a group of 4, and go past a tea tent twice a day, so everybody tends to buy 1 round of teas, but this isn't obligatory. Wear a decent set of walking boots, and it's worth carrying everything you think that you might need with you, including any spares you might need, snacks and drinks etc. If it's wet make sure that your waterproofs won't get in the way of the string (as I found out once) Above all have fun.
Which shoot is it, as I might well be there? If I am then come up and say hello and you can ask any questions that occur to you on the day.
Daniel