CX is a blast! It is also a 30 to 55 minute sufferfest of intense aerobic effort where your heart will be exploding out of your chest, your lungs will sear and your legs will be screaming at you.
Given this is your first race ever, if this is a USAC sanctioned race and they have a pure category '5' race that is the one you want to ride in as it will just be other beginner men. Barring that, look for a category 4/5 or better yet a category 4/5 35+ race. If the 40+, 50+, 60+ race is an 'open' race (meaning any category of racer can enter) I would shy away from it as there will be cat 1, 2 and 3 racers in it.
The day before the race get all your stuff ready. Make sure you have appropriate clothes for the weather plus your helmet, your shoes, water bottle with whatever sort of hydration you do, energy gels etc. If the weather is poor consider bringing a towel to dry off with and an extra complete change of clothes for post-race. Again, if it is going to be wet or muddy consider bringing a few plastic bags to put your muddy and wet shows and clothes in. Also, take your bottle cages off the bike to make it easier to shoulder in the race.
Day of, try getting to the race early in the day (ideally an hour or more before the first race of the day takes place) so you can pre-ride the course a few times. Don't try to crush it when pre-riding the course. Instead focus on learning what line to take through different sections of the course, how you are going handle off-camber sections, how you will handle run-ups and the barriers, where is there going to be a bottleneck during the first lap etc.
When you are pre-riding, if there is a section that gives you problems or you don't feel comfortable with, ride just that section multiple times until you have a game plan for it. As you are doing all this remember that for some sections the fastest solution may be to get off your bike and run with it.
While you are doing this also watch other riders. How are they handling different parts of the course? How are they dismounting/mounting their bike?
Make sure you pick up your number before registration closes for your race. Make sure you pin it on the correct side. Use 6 pins. Don't pin through the holes in the numbers.
Make sure you are warmed up before the start of the race. After you've warmed up hydrate. If your normally consume gels, gummies or other energy food while cycling consume your normal dose while hydrating. If you don't normally consume gels, don't do so now.
Show up to race staging 10 to 15 minutes before the start of your race. If it is cold or wet outside try to stay warm and dry by wearing a jacket to staging and discarding it when the officials start staging the race. Listen to what the official say prior to the race start.
During the race, as Glenn Gervis says in his answer, you most likely won't win, you might be lapped but that is ok. Go out and do the best you can, don't fret the barriers and run-ups, if you crash, skid out get run into don't panic, just pick yourself out and continue. When you put your bike down after going over a barrier or shouldering it on a run-up make sure you place it back on the ground gently so you don't drop the chain.
If you find yourself a ways back in the race focus on the closest person in front of you and make it a race between you and that person. Where does he slow down? Maybe he is slowly riding that muddy off-camber uphill section and you can run it and pick up time on him. Once you pass him switch your focus on the next person in front of you. When you are right behind someone remember pass, don't pace.
If the leaders come up behind you don't interfere with their progress. At the same time that doesn't mean you need to stop. Just kind of pull off to the side a little and let them by.
Make sure you check your results as soon as they are posted. If you think they are wrong, calmly talk to the officials about it.