This will depend upon the circumstances in which your puppy was foaming at the mouth.
I would start with, what breed is your puppy? Certain breeds, such as St. Bernards, will naturally create a lot of saliva that can turn into bubbles and look like the puppy is foaming at the mouth. This is natural and will continue for the rest of the dog's life.
Second, was your puppy highly excited at the time? The more energetic your puppy is, the more likely it is that he/she could create saliva bubbles. This is also normal, and will go away when your puppy calms down.
Third, was your puppy out exploring on his/her own without supervision? Puppies can try eating things like toads and noxious plants that can cause excessive salivation; if the bubbles don't stop within a half hour, call your veterinarian for more advice.
Fourth, is your puppy showing any other signs of illness, like harsh breathing sounds, vomiting, diarrhea or lack of energy? If so, this may be a medical emergency and I would suggest a trip to the veterinarian immediately.
Fifth, and highly unlikely, your puppy may have rabies - foaming at the mouth is a classic sign of this disease. If this is the case, you need to take your puppy to a veterinarian and it will probably die in the next couple of weeks. You need your veterinarian to collect samples to test for rabies if this happens (the testing is free, incidentally) so that you and your family can be treated for exposure to prevent you dying from rabies as well.
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The Dog might have eaten a bombardier beetle, been stung by a scorpion , eaten some detergent, or worse. What is almost guaranteed is some sort of irritant has gotten to into his mouth and the foaming and shaking is the puppy trying to expel the irritant. I recommend a visit to your vet, but keep a very close eye on your puppy for anything that might suggest poison, like loss of balance, seizures, dilated pupils, etc
Your puppy could have tasted something that he didn't like,he could have something stuck in his mouth he could have a tooth that is bothering him You should seek veterinary attention.
Call a vet NOW. Your puppy could be in serious danger, or it could be nothing serious, but coming online to ask is not a good idea when it COULD be an emergency.