I tend to have shakey lines and can't get dimensions right, any tips on how to fix those? And on how to start drawing cars properly
Practice drawing straight lines, vertical and horizontal in one swoop. No stop-and-correcting. That's a bad habit of people who lack confidence in themselves. Draw with your arm, not your wrist. When you think about the mechanics of your wrist, if the arm doesn't move, the only way your hand will draw a line of any length is to rotate your wrist... and
rotating in of itself is not a good way to get something
straight. What you want to do is keep your wrist still and use your entire arm to move, like a computerized machine that moves in only on an X/Y axis. If it helps, use your entire upper torso in combination with your arm if you're doing long lines.
Once you got down, practice controlled curves. You can practice by trying to draw a car with curves. Again, try to keep it to your arm only. Best if you can keep up that habit. Personally, if the turn of my wrist can draw out a specific curve accurately, I'll make an exception for that rule.
One other biggie when drawing cars:
drawing ellipses. Practice drawing circles, ellipses, no matter what. They are hella hard to keep consistent if you don't practice, and any car drawing turns to poop if those ellipses aren't convincing. It alone separates amateurs from the pros, regardless of how well-rendered the drawing may be.
Perspective... that's one you should know. Learning the science of it will help you understand why things look the way they do foreshortened, but to me, you need to have a knack for knowing how much to foreshorten to create the illusion of depth. I still struggle at this myself with certain angles. Look up car photos in all sorts of interesting angles and examine how perspective alters the appearance of the body. Try drawing that car. Try a different photo after until you start getting the hang of it.