There is lots of literature on dehydrating, including Sarbar's website mentioned above and the booklet that will undoubtedly accompany your dehydrator.

Veggies need to be at least blanched first, which involves cooking (preferably steaming) for 1-3 minutes depending on the particular veggie. I find it simpler (being a lazy person at heart) to use frozen veggies, since they have been prepared and blanched before freezing. I partially cook them in the microwave, just enough to defrost and bring to boiling temp to kill any germs, and spread them onto the dehydrator tray. As noted, some veggies work better than others (forget peas!). Before dehydrating big batches, do just a few and try rehydrating them at home under camp conditions (i.e. not in the microwave!). That's true of almost everything. There's nothing more horrible than going to all that work for something that either doesn't rehydrate (peas) or tastes terrible to you.

I find that chopped spinach dehydrates well and can be added to almost any dish for added nutrition. It basically becomes a powder and does not have a strong flavor. As with everything food-related, though, Your Mileage May Vary!

Parchment paper is a necessary accessory to your dehydrator. Except for very coarse food, you'll want to line the trays.