new, need advice

For a Sprint event, the biking will probably be less than 15 miles.  I'd get yourself a bike if you don't have one already.  Any bike will do, doesn't have to be anything special.  Lots of folks start out with mountain bikes and you won't be the only one in the transition area with one.  Take your bike out for a half dozen rides or so, or whatever you can squeeze time in.  Make sure before race day that you're comfortable doing the distance.  Don't worry about how fast you are or are not on the bike.  Lots of more experienced cyclists will pass you...forget about them.  Just ride within yourself, not so hard that you feel shattered by the start of the run.  Then go out on the run and with your track speed, pass all those cyclists that previously blew by you.  The motivation from doing that will get you to the finish line.

There...you completed your first race, the butterflies are gone (or at least greatly diminished) and you'll be pumped up to do it again.  Above all, have fun!

welcome to BT

you're fortunate to have begun your tri training at a young age. i just started last summer and high school was, well, a while ago for me.

how many miles a week do you run? could you add bike workouts to that and still maintain your energy for running track?

You are already doing track workouts that are intense and speed and it seems like that is your focus right now. So what to do about the bike.

DON'T worry about speed on the bike. Just take it out and ride. Start with 30 minutes and work your way up to an hour (if you have time.) Just an easy pace. Focus on spinning (same pedaling cadence as your running cadence) getting comfortable with the gears and so on, and just getting your muscles used to riding. Riding can be a recovery from running as long as you are not doing it high intensity because it is MUCH more low impact.

IF you can work it into your schedule a few bricks would also be helpful. This is running right after riding. It doesn't have to be for long, but gets you used to the transition between biking and running. Say, you do an easy 30 minute ride and then go run for 10 minutes. Really, that should be enough to get accustomed to it, as you are already a runner.

Since your track season ends 2 weeks before the tri, I'd ride the bike 3x the first week and then the 2nd week take it easy with maybe one or two intensity sessions on the bike. Get out there and do some accelerations. Don't ride more than 30 minutes at a time. Then, just go out and have fun at the race.

On the bike keep your effort level moderate, and expect that when you start to run it's going to take a few minutes to switch over. Then blow it all out on the run and see how many people you can catch!

Back in High school, I trained for a sprint tri and did track (my event was the 400). 

If you decide to do it, you'll be able to pull it off, just realize that your track times might suffer becuase you're not putting all your effort into it.  Same goes for the Tri, it might not be your best becuase you're trying to do 2 different things.

Listen to your body, and not knowing much of your back ground, you'll probably do fine. Remember to rest before and after the race!

 I recommend bricks to get your legs used to the transition since you are using a different set of muscles.  Happy training!  You'll do well.