In order to get a Greenlight Card for yourself, your parent must first complete their registration process.
If your parent has already completed registration, and added you as a child, you can download the app and sign in with your phone number. Please keep in mind that if your parent didn't include your phone number when they signed you up as their child, they will have to first "enable app access" for you (which is a menu option in their Greenlight app).
If your parent hasn't registered yet, you can register yourself by downloading the Greenlight Card app from the Google Play or App Store, completing registration as a child, and inviting your parent to complete their registration.
After your parent completes their registration, your card will automatically be created and sent out to you. Cards typically take 5-7 business days to arrive.