The sit and stay commands should be taught together at the same time. If you think about it, when we ask our dog to sit we also want them to stay where they are at, at least for a little while, it also helps when teaching a pup not to jump. These commands will usally be understood by your dog quite quickly and they will learn these two commands much faster then some other commands like leave it or getting them to stop chewing. 

 

Let's start with sit. What you need to do first is to attract the dogs attention with something (treat/toy) it likes. Take the object and dangle it right above its nose, such that the dog is forced to look up. With the dog attention firmly on the object, move your hand slowly and gradually backwards over its head. Don't be too fast or slow or else the dog shall lose interest in the object. Now, the dog will have to sit to see the object; while it is lowering its back to sit, speak out Sit and encourage it to sit. After the dog sits, reward it with praises, or a treat. Once the dog learns to sit quickly, a simple hand movement over its head is enough to make it sit.

 

Once your dog has mastered the sit command it's time to teach him to stay. Put your dog in the sit position. Say stay and take about 3 steps back. Wait about 3 seconds and if he stayed tell him to come and praise him and give him a treat. Keep doing this exercise but add a step back and about 3 more seconds each time. Only add the step back and the time after he has mastered the prior step. In no time at all you will have a dog that stays for as long as you want him to.