Good for you considering joining.
I got sworn in just a few hours ago actually.
As for the fitness level goes, staying active, eating healthy, and getting 7-9 hours of good sleep is very important. I've been training for a 5k sub 15 minutes so cardio is something I've been doing a lot of research on and I've found some interesting things.
1) Professional athletes devote around 75% of their time to low level aerobic exercise. For me right now, that means a fast walk. Around 20% of the time running really hard interval based training (all out sprints for 30 seconds or so). And only 5% at their race pace. This method requires a lot of time and not everyone (I imagine you are at school most of the day) has the time for it. Instead, do that interval training mentioned earlier 3x week. Start with 4x 30 seconds sprint with 4 minute rest between each sprint. And that's it. Obviously warm up and cool down with dynamic stretching. The rest of the week stay relatively active. In regards to your sprints, you should work toward 10x 30 second sprints with 2:30 between sprints. It's OK if this takes for ever.
2) Endurance is governed by 4 principles: Your heart rate, stroke volume (how much blood is pumped each heart beat), heart contractility (how fast your heart pushes blood out), and a preload (how much blood is pushed into your heart). Heart rate is increased over time generally, but sprints and Olympic weight lifting help out a lot (do NOT do Olympic weightlifting without proper form and supervision. Hire a personal trainer). Stroke volume is slightly dependent on your preload, but again, sprints and cardio help this a lot (mostly the sprints), same with the contractility. Finally, your preload. This is when your muscles contract and push blood out and back toward your heart. Obviously, the bigger and more efficient the muscle, the more blood gets pushed back. Examining all this, you can see that your heart can pump like no tomorrow, but not enough blood gets to it, it doesn't matter. Which leads to....
3) STRENGTH: You need to be strong. Focus on having good form. Use weights. Keep track of your weights. For building strength, do 12-15 reps 3-5 sets, at around 65% of the maximum weight you can use with 30-60 seconds rest between sets. For muscular coordination, do about 4-8 reps 2-4 sets at about 90-95% of the max weight you can lift with 2-3 minutes between sets (if you have space, do dynamic stretching and balance drills, you can find plenty on the internet). And look into "German volume training." This basically increases the volume of your muscles really fast which makes your preload better. Makes you look beast too. Make sure you are in good shape before you do this though and preferably not when you're so young.
That's the gyst of training. There's dieting, recovery, and lifestyle, but that's a whole other can of worms. Send me a message if you are looking for the info though.
Good luck!