My guess is that you can probably expect an orientation first. All the same, it really depends on the group and the nature of the retreat. If it's a formal sesshin, you've got a lot to expect. For one, the energy will be amazing. Zen is sesshin and sesshin is Zen. There are no words that can describe what the experience is like! As for the more mundane - there will be no talking. These retreats are silent, but if you have a question about what you should be doing for a work period or what have you, you aren't violating any rules if you approach a senior student with a question. Also regarding the talking bit - you will be given a chance to talk to the guy in charge during dokusan. Try to save your practice related questions for then. Two, your legs will hurt...a lot!!! Expect this from the get go, but don't be afraid of it (easy to say, right??). Stretching a bit helps purge your muscles of the lactic acid that builds up. Oh, and during meal times, if they give you a yellowish looking pickle, don't eat it! You use that to wash your bowl. You can eat it after. ;-)
All in all, just do the the best you can. No one sits like a stone on their first retreat. It takes years of practice to get to that point. Also, don't expect any earth shattering revelations or anything of the sort. The world will seem to move a lot faster once you come back home, though. This will last a couple of days. It will also be a bit easier dealing with stressful situations, your mind will a bit clearer, and you'll just be kinda happy for no reason. That should whet your appetite to go back!