If you have a puppy, you must start training early, but do it gently.
Your puppy has to have confidence in you before he can begin learning. Remember that puppies are like children - they have growing bodies, short attention spans, and will only learn things when the conditions are right and when they understand what it is you're trying to teach them.
That said, the earlier you start training dogs, the better. More specifically, it's best if you start “socialization” from 8 weeks, simple commands like “come” and “sit” from 12 weeks, and more intensive training at 5 to 6 months.

While some early training can be started as soon as you bring your puppy home, the optimum time to begin obedience training is somewhere around 9 to 12 weeks of age.
Keep in mind that training can cover a broad range of topics – I’m not suggesting that you begin training your puppy at 8 weeks of age for agility competitions! Your training should start off with the basics – teaching him “No!” and beginning house-training.
Socialization skills are next – experts tell us the best window for your puppy to learn socialization skills is between 3 and 16 weeks – that’s the best time to insure that your puppy grows into a well-adjusted adult.
And remember, socialization isn’t about teaching him the right fork to use at the dinner table – it’s about giving your dog the self-assurance to deal correctly with any social environment he finds himself in is one of the most valuable and lasting lessons you can teach him.
A well-socialized dog will interact well with all types of people and situations, even those he has never been in before. With appropriate social skills, your dog will show little or no fear of most objects, people or other animals, and even if startled, will recover quickly and won’t panic.
Bottom line, a well-adjusted dog is one that is comfortable in a variety of situations and surroundings. He may be excited in a new setting, but not fearful. The key here is to create positive experiences as you expose your dog to more and more new situations.
Even training your puppy for 5 – 10 minutes per day as soon as you bring him home will make a big difference in the social skills and adaptability of your puppy.
Keep in mind that puppies have very short attention spans, so keep your lessons short and fun. How short an attention span? That depends on the age of the puppy, his breed and how mature your individual puppy is – but a good rule of thumb is to keep the training sessions within that 5 -10 minute range.
Depending on your puppy’s age and maturity level, sometime between 3 and 6 months of age you should be moving the training into the area of the basic commands such as Sit, Heel, Down, etc.
It’s important you have realistic expectations about your dog’s capabilities at this point – I don’t expect a puppy to be responding to the basic commands with any degree of regularity until they’ve reached 6 months of age.