When giving commands to a dog, a calm, firm, authoritative voice is most effective. Dogs do not respond well to hesitant, pleading voices, nor to yelling, which might sound to the dog like threatening barking or scolding. It is also important that the word used for the command and the pitch of the voice be consistent each time the command is delivered so that the dog can more easily learn what the owner means (siiiiiiiiiiiit does not sound the same as sit, for example).
Using the puppy’s name before a command ensures that the dog knows that a command is coming, that it is for him (rather than for other dogs, children, or people), and that he should pay attention. This is important because dogs hear a lot of human speech that has no relevance for them at all, and it is easy for them to disregard commands amongst the babble.
To reinforce the command, the dog always gets some kind of reward or reinforcement (praise and usually a treat or toy) when it performs the action correctly. This helps the dog to understand that he has done a good thing.
Note that not all dogs are trained to voice commands. Many working breeds of dogs are not trained to a voice command at all; they are taught to obey a combination of whistles and hand signals. Deaf dogs are perfectly capable of learning to obey visual signals alone. Many obedience classes teach hand signals for common commands in addition to voice signals; these signals can be useful in quiet situations, at a distance, and in advanced obedience competitions.
The specific command words are not important, although common words in English include sit, down, come, and stay. Short, clear words that are easily understood by other humans are generally recommended; that way, people will understand what a handler is telling his dog to do and other handlers have a good chance of controlling someone else’s dog if necessary. In fact, dogs can learn commands in any language or other communications medium, including whistles, mouth sounds, hand gestures, and so forth.