There are calculators on the internet, but I'd strongly suggest using this only for research purposes, if at all.
I'm seventeen and otherwise healthy (other than my asthma), and my best peak flow is still quite a bit lower than what is considered normal.
To determine your personal best, doing peak flow readings in the morning when you get up and evening when you go to bed for two weeks can help you determine your personal best. After you determine your personal best, you can use this as a guideline for your lung function.
Once you've established a personal best, it's always good to check with your doctor to see what to do next, too.