Diane,
Welcome! Hope you enjoy your new WSM as much as I like mine.
Fill the pan with about a gallon of water. If you're using the Minion Method (see elsewhere on this site) that should last 6-8 hours. Replenish as needed. I just did a pork but which cooked for 18 hours. I started with a gallon and added 2 quarts of water during the process. No big deal.
It helps in the cleanup to foil the pan first. You'll need the extra-wide foil. Do the bottom first and then line the water side. If you do in the opposite order you might wick water out of the pan and have a mess on your hands. This, too, is described somewhere around here.
Last of all, some recommend the use of sand instead of water. I have not tried this yet, but there's no reason it shouldn't work.
Good luck!