Unfortunately, for many dogs a car ride is not a happy occasion. Many simply can't stomach the motion and become nauseous while traveling. Many become so worried that unless they can weave back and forth during the trip (not a safe or practical way to travel), they whine and howl until they are “freed” from the moving vehicle.
Some foam at the mouth, vomit, become extremely nervous, and can even become destructive (to the car, other passengers, or themselves). Because there are times when it's necessary for a dog to travel in a vehicle to another destination (such as the veterinarian), if this describes your dog, you need to help her get over her fears.
Not traveling well is the result of either not being properly exposed to a car from puppy hood or having a bad experience in the vehicle. Either way, it's your job to reverse the negative associations to positive ones. This takes time and patience and can be frustrating.
If you are frustrated by your pup's inability to ride in the car without incident, consult a professional trainer. It will help you and your puppy to have someone who can approach the situation unemotionally, and who can coach you through the process while targeting the things that may be upsetting your pup.
Before a dog can relax in the car, she needs to feel comfortable near the car. Using treats, reward her for not just approaching but being able to stay near the car. Leave the car door open so she can look into it, but don't in any way force her into the vehicle. Every day, for just a few minutes a day, make being near and getting into the car rewarding and safe for your dog.
Once she's in the car, secure her with a seat belt, close the door, and get in the driver's seat. Don't go anywhere, just pet her and give her a treat or two for just being in the car. Slowly, slowly, work up to starting the car and making very short, then progressively longer trips, always ending on a pleasant note.
If your dog is fine getting into the car but only gets carsick or upset as the trip goes on, you will need to adjust your strategy. You will need to make some trips that end at the first signs of upset. So for example, get in the car with your dog as usual and begin to drive. As soon as you notice your dog looking “off,” pull over, take her by the leash and collar, and get her out of the car. The fresh air and new smells should divert and calm her so she won't be sick. After a few minutes, and with your reassurance, put her back in the car and start driving again. Drive in the direction of home, and if she makes it all the way, give air, act subdued, and go into the house. Working this way, your dog can learn that not all car rides end badly.