The first and most basic command you could start with is Sit.
Having the dog infront of you and with a treat in your hand, gently move your hand pass the nose and
over the head.
The response from the dog (who wants the treat) is to lift is head and then sit down.
If needed you can apply a gently pressure to the butt as well.
When you have done this a couple of time, you can add the word Sit.
Next step is to not use the treat to shape the behaviour, so you just stand still and say the word and praise and give treats as a reward after they sit down.