I used to do some near-stock road racing racing in the U.S. and based on that experience I'll say this:
Twenty minutes isn't much time; however, depending on how hard you drive your car it's plenty of time to boil your brake fluid and use up tire life equal to many thousands of miles of highway driving. If you are not planning to race your car, then why not take a sensible approach and resist the urge to produce the absolute minimum lap time? You can have plenty of fun using a longer braking zone; i.e. less brutal braking. You can still maintain high cornering speed, hit the apex correctly, etc. and learn nearly as much as you can learn by driving as if you were in a genuine race. That way you won't over-tax your standard brake system, which is quite easy to do especially with such a heavy car.
You might consider putting in high performance brake fluid and racing brake pads, but I wouldn't do that just for 20 minutes of track time. But, if your brake fluid is old, this might be a good excuse to change it since most of us don't change it as regularly as we should. That would also force you to bleed the system, which is a good idea. I'd leave the pads alone.
I would over inflate my tires, especially on the front and then consult with your instructor when you get to the track and reduce the pressure to what he recommends. It's easy to let excessive air out; sometimes it's hard to get more in. You'll definitely want more pressure than you usually run on the street. Paint some white shoe polish on the tire shoulder to see how much "tire tuck" you get during your session. You won't have time to adjust tire pressure during your stint, but it will give you a data point in case you get hooked and decide to return for another track day.
When you get to the track you should take everything
out of your car including the spare tire, jack, junk in the glove box, floor mats, etc.
You'll most likely wash and detail your car. DO NOT get any plastic protector on your steering wheel or foot pedals; it's slick.
I assume you'll be wearing a helmet. If so, do a little driving with a helmet before-hand to be sure you can adjust the seat properly even at the risk of appearing silly to your neighbors.
Practice your footwork on the street too, but be careful. It's pretty easy to screw up when you're learning. Wear appropriate shoes, narrow and thin soles are good. Practice some version of "heel and toe" which suits you. That might be toe on brake and heel on throttle, or perhaps the other way around. You might consider learning left-foot-braking. It really helps, but this is something you don't want to try for the first time in heavy traffic during the morning commute to work. You'll need to need to learn how to switch from left foot braking to heel-and-toe and back again without thinking about it and that definitely takes training. Be careful. Your overall track day cost will go WAY up if you rear-end someone on the street while practicing left foot braking.
I add this to what Chris said. He recommends "brake or steer" but not both. In fact, trail braking, is not only a mandatory racing technique, but valuable on the street for those times you find yourself entering a corner faster than you anticipated. In short, trail braking is the art of easing off (but not fully releasing) the brakes as you turn-in so as to share (for a period of time at least) the available tire-to-pavement traction between turning and slowing down. A tire can produce only so much traction. You can use it all for braking or all for turning or the traction can be split between the two. Ask your instructor. He/she may agree with Chris and recommend that trail braking should be reserved for more advanced training.
Finally, buy a book about how to drive a race car and read it. Then read it again. HERE
is one example, but there are many good ones available including the classic "The Technique of Motor Racing," by Piero Taruffi. It's very old, but still one of the best books about how to properly take a car through a corner.
Oh yeah............. think about how you're going to get home if something goes wrong. I've seen plenty of cars wadded up on track days. If you crash yours and go home in a taxi, be sure to tell your insurance company you were taking an advanced driving safety course with a qualified instructor. Hint: Don't use the word "race".