I'm assuming you mean social security disability. What you have to do is apply and qualify for it. Depending on how severe or mild your current stage of CF is, you may or may not qualify. You and your doctor need to work together to apply for it. You qualify based on how much you have been recently hospitalized and how often you're on antibiotics. You don't necessarily get it just for having CF (unless you live in California, you can apply for GHPP, but that's different.)
You can call the CF Legal hotline if you have questions; I'm sure there are others here who know how the system works too.