HERE ARE THREE BEST THINGS YOU MUST DO TO REALLY IMPROVE YOUR RUNNING
1.Contact your doctor, and see how you should train with your asthma
2.Join a running club
- Get a highly reccommended book on running
My last 1 mile time was 5min exactly (done a while ago) but I'm 17 now and I have not trained for it for so long that I could probably improve if I started training again
Sorry mate your question is asking the wrong website, but If you want to get better at running then this is what I would do: pratise hill sprints, Fartlek (running normal speed, then increasing, like H.I.I.T). I also found barefoot offroad running (on courses I could trust) extremely useful. Have 3-4 square (not average American size, a few hundred calories). Energise with some light carb's before and after a run (a banana, or similar, would be perfect). Revving High is right you should work out what is weaker your lungs or muscles, the former being a sign of how much cardiovascular fitness you have (it's most likely your muscles at this age, but with your asthma perhaps not) and train whichever ones weaker up with specific exercises. Check your mile time every week (preferably on a flat road) and see how much you have improved.
If you want a proper program then please contact me back, I look forward to hearing from you.