Getting used to the car
A few times throughout the day, walk out, sprinkle some treats on the ground near the car or give them to your pup by hand, then just walk away. Also feed them their dinner close to the car. Repeat until your pup is happy to walk to the car with you.
Next, start walking out and sprinkling treats into the car for your pup to reach their nose in to get (at this point put them close enough to the edge that they don’t need to jump in to get them). Repeat until they are comfortable retrieving the treats in this way. Gradually start to place the treats further and further into the car, so that your pup has to go in to get them (they can jump in or be helped by you if they are too small). Then just let them come out again whenever they choose. Once they are comfortable in the car, start giving them their breakfast/dinner in there.
Next, you can start to shut the boot/crate/car door for 1 second, then immediately reopen it and let them out. Gradually increase the time it is shut. Once they are comfortable with being in the boot for 30 seconds, start also turning the engine on during this time. Then just open the boot and let them out again.
Finally, start taking them on very short journeys. These can start as being round the block to back home, then progress to going somewhere very close for a walk, then further away for a walk. All journeys must end in doing something your pup likes at this stage (e.g. going for a walk). With practice, you can increase the time that your pup is in the car for journeys. Make sure that they have lots of happy car trips rather than just trips to the vet!