You're 15. You're not getting a new car. You don't know how much twenty grand is. You're not going to save your money for nothing other than a car, nor are you considering the cars service, repairs, maintenance, insurance (since your 15, will be sky high), and other various costs a car will need. You're also a new driver so an accident for you is probable.
Keep dreaming.
Im only 15 and Im considering these things, Im not sure why exactly you're not.