This is a completely subjective question. It's "okay" to compete in as many events as your coach allows. In terms of competition results, you'll do best if you pick an event that you have the most natural ability in and train like a maniac for it.
If you decide to combine activities, say the 5000 meters and the javelin, you will have to split time between them and won't be able to achieve your best in either activity. But if you're having more fun that way, go for it!
It's not just "track" or "field" though. When you say JUST track, you are saying (not including relays):
100 m, 200 m, 400 m, 800 m, 1500 m, 3000 m, 5000 m, 10000 m, 110 m hurdles (or 100 m hurdles for women), 400 m hurdles, 3000 m steeplechase.
If you like suffering, the 400 m is probably the most difficult running event. It's considered a sprint, but the human body can only carry speed (i.e. go anerobic) for about 60 s before slowing down, and you have to work up to that, which requires lots of suffering. Go out and run as fast as you can for as long as you can. Unless you have been in track for a while, you'll be gassed well before 15 s have gone by. A fast high-schooler can run the 400 m in the high 40s or low 50s. This is a long time.
Keep in mind that Usain Bolt could destroy the world if he trained for and ran the 400 m. He'd break the world record by seconds. His talent is ridiculous, unprecedented. But he's too lazy to do the work! And Usain Bolt isn't lazy!
If you like even more suffering, do the 400 m hurdles.
When you say JUST field, there are also many choices:
long jump, triple jump, high jump, pole vault, hammer throw, javelin throw, shot put, discus, and jousting.
Okay, I'm kidding about jousting.
Pole vault appears to require the most balanced physical fitness and complicated technique. You have to be fast, have lots of upper-body strength, and be willing to learn very complicated technique to do well.
Keep in mind that some people do a few running events and no field events, and vice-versa. What you should do if you're in high school and just checking out track and field is go to a meet and watch as many events as you can. Try the ones that look fun -- that look like you'd be willing to put 100% of your effort into. If running looks fun to you, try some sprints, try a 1500 m. If you're slow in sprints but hold up better over distance, maybe stick with the 1500 m or longer. (The 5 km and 10 km are not run in high school, though.)
If you like to run but want to do something that requires much more technique, try the hurdles or one of the jumping events. If you like jumping or throwing, do some field events.
Keep in mind that the more events you try, the better overall fitness you will achieve, but the less you'll be able to achieve your personal best in any one event you try.
Also, at the high school level, natural born sprinters win all the running events, even up to the 3000 m. The reason is that speed cannot be acquired through training -- it's God-given. Endurance, however, can be learned. You can acutally train your sprinting muscles to perform long-distance activities. So the really good 1500 m and 3000 m runners are sprinters who prefer to run long distances. When they get closer to the finish line, if anybody is still near them, they can kick it into high gear, go anerobic, and blow all the slower people away.
In short, do what you feel like doing! If it's too much for your body, your body will let you know. Trust me!