There is basically 1 thing you need to know, stay calm and authoritive.

What you need:
5 minutes a day
handful of small treats
leash (optional)

Sit:

Most dogs will almost instinctively sit down once you hold out a treat because their nose will point towards the smell of the treat. Now, hold up the treat and command the dog to "sit" in a calm but firm tone. When the dog sits - praise! praise! praise! If the dog hasn't found the position yet, gently push down on his hind to get him into place. Once he's down - praise him! Always and only use the same one word for each command. Remember, your dog wants to please you so make it easy for him.

Down:

To teach the "down" command, place your dog in the sit position. While holding a treat, tell him "down" as you lower the treat to the floor between his front legs. Gradually bring the treat back to you so that his paws come down and he goes into position. when he's down - praise! praise! praise! "good down" and give him a treat. The sooner you start to practice these commands the sooner and easier you'll have a well trained dog.

Come:

This command is best taught with your dog on a leash and with another person. Have your dog sit and then tell him to "come". If he doesn't budge or runs the wrong way, give a tug on the leash while commanding him to come again. Remember to talk firm, but affirmative. When he gets to you, praise him with a "good come" and a treat. Continue practicing for a few minutes because after that your dog will be bored and distracted.

Practice these commands for a few minutes at a time a few times every day. Even older dogs love to please and this will help to exercise their mind a bit.