Hi. You don't "just get it" as you get older. You are born with it. It is a genetic disease and part of your genetic makeup, where both parents have to be carriers of that gene. The disease may be very mild when you are born, so it is misdiagnosed as just asthma or not diagnosed at all, and then can progress as you do, but you don't just "get it." You always have it.
If you think you might have CF, then I agree, you need to go to your doctor and you need to tell your parents. The tests are absolutely painless. The sooner you are diagnosed then the sooner you can start treatments to prolong your life and keep you healthy. And if you have siblings, they should be tested as well, if you test positive, so they can also get treatment.
Let us know what the results are and we will help you work through it as best as we can.