Hopefully, your buddies will have extra clothing you can borrow so you don't have to buy the stuff. The cost of winter gear can add up very quickly. There are places that do rent jackets, pants, gloves and helmets, but I would recommend at least getting some wool long johns and a wool long sleeve shirt to wear under the jacket.
As for actual gear, just rent the equipment. The shop will fit you properly for boots and a board. Also, be prepared to fall a LOT if you have never snowboarded before. I've only skied a few times and have never boarded, but the snowboarders who were taking lessons were always falling down. Usually, gear can be rented for 25 -30 bucks for the day.
Hopefully this helps and have fun!!