If someone ask you a question in a affirmative way you answer it with "oui".
Ex: Voulez vous dormir? Oui, je veux.
Do you want to go to sleep? Yes I do want to.
If the same question is done in a negative way , you answer it with "si" instead.
Voulez ne voulez pas dormir? Si, je veux dormir.
Don't you want to go to sleep? Yes I do want to sleep.