While it's still a few months old, take the puppy out and sit with it in the car. There's no need to start the engine yet. Just let the animal get used to the sights and smells of the automobile. Later there will be less cause for alarm. Many dogs see the inside of a car only on those rare occasions they are driven to the veterinarian.
Few dogs enjoy that particular experience.
After a few sessions of sitting in the car, the puppy is ready for short trips. Plan to take your puppy out before eating, so it has an empty stomach. You may want to begin by circling the block a few times. Soothe the puppy's excitement or anxiety during these rides, and reward afterwards with praise and treats. Before long, the puppy should think car rides are another part of normal life, and it will be ready for extended trips without incident.