1. Try to rent your gear in advance if possible. Some ski/snowboard shops near resorts are happy to accommodate such requests. If you rent from the resort, give yourself plenty of time to do it. Don't let yourself be rushed. Make sure the boots fit reasonably well and that you understand how to operate the bindings. If possible, ask them to adjust the bindings to make the stance more comfortable for you. If helmets are available to rent, rent one.
2. You should buy in advance: snowboard or ski gloves, snowboard socks (one or two pairs will do to start), snowboard pants, wrist guards, and soft knee pads. (You'll spend a fair amount of time on your knees at first.) I assume you already have some kind of suitable jacket. Don't worry about fancy base layers or long underwear at first; you won't be heading into a frigid zone, hopefully. Test the gloves with the wrist guards to make sure the gloves fit over the guards. If you can't rent a helmet, buy one. It's very easy to give your head a solid whack from a body slam suffered at a low speed. Oh, and padded shorts are optional.
3. If your scheduling is flexible, choose a day with good weather -- temp in the 20s (F), clear skies, recent snow, no recent rain. Ice is your enemy.
4. If you can afford it, take at least one private lesson, preferably several. Group lessons are less effective, and could quite poor. However, even a group lesson is better than no lesson at all. Snowboarding is not really an instinctive sport, and you'll have no "crutches" (poles, training wheels, skating rink walls, etc.) to hang onto while you're developing your balance. Understanding edge technique is the key, and a decent lesson will help a lot.
5. Buy your own board, boots and bindings as soon as you're ready to make a commitment to the sport. You will progress more quickly with consistent gear of good quality.