I think helmets are an absolute necessity, even if you don’t ride aggressively. There are lots of collisions up there. It seems to me that people at the So. Cal. resorts are more reckless than anywhere else. Ambulances and paramedics are always seen hauling people out.
Common snowboarding injuries are broken tailbones, broken pelvis (both from falling on ass), and broken thumbs/fingers/wrists from falling with outstretched hands. Learn to fall without hurting these areas and you’ll do well!
With this great snow it’s the best time for you to get out there. The chains are easy but you’ll get a little dirty putting them on so bring a towel and some sanitary lotion.
There are plenty of how-to snowboard videos on youtube that will help you get started. I strongly suggest you review things like “how to get off the lift” before you get going. Then you can move on to your front-side 360 (ha haaaa NOT).