If you recently got a puppy, or you're considering getting one, your vet will tell you the puppy shouldn't be in close contact with other dogs until it's around 4 months old, which is when it gets its first rabies vaccination. This is the reason most group obedience classes don't allow puppies to join until they're at least 16 weeks.
However, obedience training should start sooner than 4 months of age. I offer private, in-home dog training, so it doesn't matter where in the vaccination schedule your puppy is. The sooner, the better!