Summarize this article in one sentence.
In order to have a well-trained guard dog, you will need to start socializing your German shepherd from a young age.  Puppies are more receptive to new things until they are about 18 weeks old. During this time, the things that they see, hear, and smell are less frightening. It is important to make sure that your German shepherd puppy is well-socialized during this time so that he will confident rather than anxious or fearful. To socialize your puppy, make sure that you expose the puppy to all sorts of different situations. Walk him past schools when kids are around, walk beside busy roads, take him out in the rain to see people with umbrellas, expose him to a vacuum cleaner and hair dryer, and introduce him to lots of other types of dogs and people. Try to make the experiences as wide and varied as you can. To teach your German shepherd to be a guard dog, he will need to be well-behaved and you will need to train with him every day. The best way to go about this is to find a good trainer or obedience course to help you. Look for a dog trainer or training class in your area that uses modern, well-informed methods of training based on either clicker training or reward-based training.   Clicker training is a method of training that marks positive behavior with a clicking sound. Trainers who use this method also use rewards such as treats and praise, but the clicker helps to mark good behavior. Reward-based training uses praise, treats, and other things that will motivate your dog to follow your commands. Do not use old-fashioned methods of dominating a dog, such as alpha-rolls or scruffing techniques. These methods are outdated and ineffective. Before your German shepherd can become a guard dog he needs to learn how to listen and respond to your commands. That is why it is a good idea to start with basic training commands such as sit, stay, and come back. Once your German shepherd has mastered basic commands, he will be ready to take on more challenging ones. Make sure your dog is well cared for, fed good quality food, and receives plenty of exercise. You want the dog to be fit and mentally stimulated. A bored, out of condition dog is more likely to overreact in the face of an intruder, which may lead to one of your friends or neighbors getting injured. Train your dog daily, ideally in two to three sessions a day. Each session varies in length depending on the attention span of the individual dog, but German shepherds are often capable of being trained for 20 minutes at a time. Your German shepherd will enjoy training more if it always ends with a treat and some praise. If your German shepherd starts to make mistakes, bring the lesson to a close with a simple command that you know the dog is capable of doing. That way you will be able to reward him and end the session on a positive note.

Summary:
Socialize your German shepherd when he is a puppy. Find a dog trainer or obedience course. Teach your German shepherd to follow basic commands. Take good care of your German shepherd. Develop a regular training routine. End each training session on a positive note.