Dogs who know how to lie down on command are far more welcome in public places and can be a lot more enjoyable to have at home. One thing to keep in mind, especially when you have a high-energy dog, it takes far more energy to stand up from the down position and most dogs will remain lying down on their own once they learn the command.
The command most people use to teach their dog to lie down is simply one word, "down" or depending on the situation "Go lie down". However, the latter is used mostly at home to get the dog to go somewhere else before he lies down. The most important thing to remember is that you are not just teaching your dog to lie down, you are teaching him a vital skill that will keep him out of trouble and out of the way of other people he is around. It is also a good command for him to know when you take him out in public places such as outdoor cafés and parks.
Teaching your dog to lie down from standing can be more difficult than teaching 'down' from a sitting position, when your dog is more likely to be focused and somewhat relaxed already. It can take several months of regular training sessions before your dog obeys the command every time you give it.
The big thing here is perseverance. Of all the things your dog needs to learn, being told to lie down is probably going to be one of his least favorite as it locks him in place until he is told to get up. While most dogs spend a large portion of their lives lying down, most don't really like being told they have to--in much the same way as a child doesn't want to be told to go take a nap. You can, of course, teach any age dog to do this, but the younger you start the faster your pup will learn.