Quite simply, you do not want an out-of-control giant dog. Training is important for all breeds, and helps establish a great bond between dog and owner. But for big dogs, it becomes a particular concern.
Training is most effectively accomplished when begun from an early age, but it is never too late to have a go. Here is some guidance for the most useful things to teach your big dog.
Have your dog stand beside you, facing forward. Raise a treat above his nose - not so close that he can grab it. Slowly move the treat back over his head, toward his tail. You should find he will move into a sit position as he moves his head back to follow the treat. Give treat as soon as he sits, and praise. Repeat a few times, then start adding the command word.
Click as soon as he sits, then treat. Don't start adding the command word until several successful repetitions have been accomplished.
Get your dog into a sit position. Now, position a treat between his front paws. Slowly move it away from him, keeping it on the ground. He should lower himself into the down position. Treat, praise, repeat. Start adding the command word after a few successful repetitions.
Click as he moves into the down position, then treat. Don't start adding the command word until several successful repetitions have been accomplished.
With your dog on a loose lead, position him by your side with the aid of a treat. Walk one or two steps forward, encouraging your dog to follow closely with the aid of the treat. Give treat after one or two steps. Repeat. Start adding the command word, and increase the number of steps you take before treating. After some successful practice, try it off the lead.
Click when your dog moves with you and treat immediately.