Man after driving Sonoma all week in iRacing FR 2.0 I would say this is a terrrrrible track to make your first. There are at least 4? Corners where if you turn in too early you're likely to actually die. And if you don't have instruction for your first track day you are making a big mistake IMO.
It gets really hot and sweaty, so I kind of wanted the open face. And I wasn't sure if I was going to continue tracking...so I got a cheaper approved helmet. My group requires either Snell SA2010 or SA2015
Seriously - You need to try them on. A poor-fitting helmet is either useless or dangerous.
I support (i.e. pay more to keep them in business) my local racer supply shop in order to insure that for those items where fit really matters, like helmets and shoes, I have the opportunity to try these items on to be sure they'll work like they should.
If you also scroll on on YouTube page I have a video of experience at Sonoma. It is true Sonoma can be a scary track if you aren?t careful - in my video two cars crash because of overly aggressive drivers. That said if you are with a good instructor and drive to comfort/limits you should be fine.
Another great video series is by Rob Dietsch who goes over car prep prior to a track day.
Be safe, remember you are going to be pushing your car in conditions more demanding that everyday street driving.
Don't rush to be the fastest around the track or others. Seriously. Take the time to get to know the car really well until the car becomes an extension of your body and you understand everything about how it drives.
As was mentioned before, track day insurance is a very smart idea, it protects your investment.
So, think of the track day as going on a date where you get to know your car better!
Ill throw my two cents in on this also but there is a lot of good advice given already.
1. As Phil said check your ego at the gate but don't be apprehensive. We all were there once, some people need reminding of this once in a while. As with any motorsports you are not going to be the fastest guy on the track. Just like autox, time trials and track days, there are fast doods there and they have been doing it a long time. Your goal is learn what you can from your instructor and reach the level they have if you so choose. Learning the line and being consistent should be your only goal first time out. Lap times are not your concern. If you have an instructor you are not learning from or have personality conflicts with ask for an instructor change. Remember the only trophy you are taking home is your car on the trailer.
2. Car prep. Everything in good working order. You are going to push your car a little harder than you ever have. She will need a little TLC before hand, and that means a small leak can turn into a bad leak quickly. Bad brakes can wear out in a day. Worn tires can make a bad day for you. A track car consumes, make sure you have enough to last you all weekend.
3. Brakes. You as a first time novice will not be pushing it hard enough to need track/race performance parts yet. Yet being the key word. I started with some Ferrodo pads and basic Motul fluid but as your game gets better you will need to move up the food chain with brake rotors, pads and fluid. Make sure you have enough pad for the weekend and fresh fluid. Fresh meaning recently flushed not recently topped off. It wont hurt your car to use a high temp race fluid, works just as good for a DD.
4. Make sure you have a half tank of gas at all times. Don't be that guy out on track that runs out and needs a tow or even worse starves it in a hard turn and quickly decelerates.
5. Take a tire pressure gauge, small air pump and torque wrench. As soon as you get off the track check your tire pressures and lower them to what the manufacturer suggests. As track temps increase and ambient temps increase in the afternoon lower the pressure 1 to 2 pounds below optimum. Check lug torque after every session. You will need the pump in the AM the next day because the pressures will lower overnight.
6. Look at your car after every session. As I said before, track driving consumes! Look at the brakes, look for leaks, check your fluid levels, look at the condition of the tires. You will have time to swap war stories over beers later. Keep a small malfunction from turning into a bad one later.
7. Study the track map a bit and don't let it be a surprise to you your first time out. Study the signal flags your organization uses.
8. This is important and several people may disagree with me. Track insurance! Its not cheap but wont break you. Most insurance companies will cover zero if you are in a mishap at an HPDE. Its better than paying for a wrecked car over the next five years.
9. Last you are paying to have fun and learn something new. No trophies, no cash purse. It is an experience and a skill. Go learn it and have a good time!