First we would like to have a look at the yes/no-questions with est-ce que. However, we can already now tell you that all kinds of questions can be formed with the construction with est-ce que.
Does he come?
¿Qu' est-ce que tu veux?
What do you want? (What is it, what you want?)
¿Qui est-ce que tu vois?
Whom do you see? (Who is it, whom you see?)
¿Qui est -ce qui l'
Who has said so? (Who is it, who has said so?)
¿Qu' est-ce qui te
What do you like? (What is it, what you like?)
¿Quand est-ce qu'
When does he come? (When is it that he comes?)
We will come back to the open questions later on. Now, please have a short look at the construction:
You take a simple declarative clause and put est-ce que in the beginning, quite easy, don't you think?
tu l' as vu? Non.
Have you seen him? No. (Is it that you have seen him? No.)
Est-ce que tu as déjà mangé?
Have you already eaten? (Is it that you have already eaten?)