How to become a test driver?

Hey I'm 19 years old and I wanted to know what would I have to do to test cars like ferrari mustang what degree do I need to have to make this happen

First the bad news, you would have to build it & not get sued by Ford & Ferrari because it doesn't exist.
Second, and this is the good news, you don't need a college education to drive an imaginary car.

Dream on!

Alright so unlike the previous 2 answers I'll attempt to be useful by actually explaining what happens with test drivers.

test drivers come primarily from a racing background. Their keen driving skills enhanced sensitivity to small changes make them ideal for helping engineers get feedback on what a car needs before they are good to distribute to the public.

a test driver might be a racing driver who is thinking about retiring after many years with a manufacturer team like VW racing, Ford Racing or Ferrari racing. Rather than just not doing anything else, the manufacturer might offer them a job as a test driver where the danger of racing isn't as present.

to get into racing, you need to start at a very young age. Most racing drivers start their careers at around the age of 8 in competitive go karting. Some will even start at the age of 4 (Max Verstappen, the youngest ever driver to score a Formula 1 contract started at 4 1/2, he is now 17 and driving for Toro Rosso).
as the drivers start to win karting championships they are then able to move up to larger cars like Formula Fords, Formula Renaults, or other GT series using road cars. If they do well in these series they will attract the attention of teams in larger series where they might score a professional driving contract. Depending on their skill and success, drivers may keep moving up to top tier levels of racing like IndyCar, World Rally Championship, World Endurance Championship or Formula 1. Others just end up sticking in these lower series.

considering you are already 19, you are already some 10 years short of driving/racing experience against other people so statistically, you won't become a test driver. In that case, it is recommended that you go through with getting an education (professional race car drivers almost always neglect their education even from high school to focus on their racing careers). I would suggest getting a degree in mechanical or automotive engineering. If you were to get a job on some racing team as a mechanic/engineer and then later went through racing school, you could do some test drive work which could get your foot in the door with a manufacturer racing team. If this is the case, you will most likely be an asset to a company because of you mechanical knowledge rather than your driving ability but you still get the opportunity to test cars out.