In reply to shaunmillar:
This shouldn't be too difficult for someone your age (17?) and who doesn't seem too overweight (your profile pic).
I would suggest you just need to get used to running a bit more and then combine this with high intensity speed sessions. I would suggest trying to run a 3 miler once a week at a reasonable pace (7 min miles), then two varied speed/technique sessions.
A speed session would involve something like 'mile repeats'. Which basically means running a mile at your required pace (6 mins), resting for a few minutes (4 minutes) and then repeating it (maybe three times). You are basically 'dialing in' this pace into your muscle memory so that your body knows how it feels, and can handle it when you're tired (3rd repeat).
A high-intensity hill session and/or technique session. So. . . either run up and down a steep hill as fast as you can, or practice running with high knees, or an extended stride. Don't forget to use your arms.
Every now and then you should drop one of these sessions in preparation for a test run (i.e. 1.5 miles at 'race pace'). You drop one of the sessions so as to give yourself a good chance to rest (a bit) and start your practice run in good condition.
Your 3 mile 'long' run should be quite slow, just getting your body used to running. The mile repeat session is the important one.
Equally important is not to get injured. If you haven't run much before then take it easy to begin with. Don't do all of the above in the first week (you will be sore and demotivated the next week), maybe just start with a gentle 2 mile jog at the weekend and 2x mile repeats, then build up to the big day.
If you're not seeing improvements in your mile times in a month, then re-think.
You should be fine.
Best of luck.