I want to design a car

Hello there, I'm glad you asked this since there's not many discussions about Automotive Design on Quora and when I was in your position, I came here for answers too and found nothing relevant.

I can only tell you about what it takes to graduate Uni, since I'm a year from that point myself and I'm studying Automotive design which focuses purely on the aesthetics and ergonomics, then making it fit the requirments for production (which is, for me, the demotivating part).

Sketching:

Sketching is a NECESSITY to becoming an automotive designer. It allows you to put your ideas into visuals for everybody to see. If I was in your shoes, or if I could go back to when I was 14, I'd sketch every single day, for hours a day to build up my skills. But it can be extremely disheartening when you create a sketch and it's not up to par with what you want it to be. This is a skill you can only develop by not getting disheartened by the quality, but keep churning out sketches like no tomorrow. (A quick tip to improve, turn your paper upside down and check if the proportions look correct, then re-draw the correct proportions and essential fix the drawing on the back of the paper, don't get economical with paper), look at other people's sketching also and how they communicate ideas.

One way to communicate volumes is by line weight. Everything from side profile, to front 3/4 and rear 3/4 perspectives MUST INCLUDE line weight to correctly depict the surfaces. For example: if the light is shining above the car, then every surface facing to the floor would be darker than the surfaces facing towards the horizon or sky. So that's a little something to practice.

Practice rendering with pencil. Now this can be particularly tricky, but I learned by looking at other people's work, and that helped me find my style. It's not something I can simply teach, because every design is different so use some basic logic to figure out where the reflections, highlights, shadows and blends are on the car.

Thirdly, when designing a car you always start with a side view because it's much easier to control the lines. When you do a side view, always do the ground line first, then the wheels. Start by determining how high the car is going to be off the ground (just under 3/4's of a wheel if it's a supercar, just above 3/4's if it's a crossover, and exactly 3/4's if it's a sedan/city car etc...) those are some basic guidelines that'll enable you to practice communicating the stance of the car.
Then, divide the wheels into 3 sections, lower 3/4, middle and upper 3/4, then design your side view with those proportions in mind. For example, a shoulder of a car is usally above the wheels, but if you wanted to create a fast looking, dynamic character line then you could start at the rear of the car, follow the line through the rear wheel upper 3/4 and then follow it through to the front wheel lower 3/4. (That's what I mean by keeping those proportions in mind, make everything meet).