Correct answer fo the question with 'do you'
Solution 1:
The rule of thumb: If the question begins "Do you..." then the answer is "I do." If the question begins "Will you..." then the answer is "I will."
If you are so nervous at your wedding that you cannot pay attention to the ritual questions you are asked, then just remembering the first two words of the question will get you through without too much trouble.
In the US, we do not answer "I want". But I believe elsewhere you may sometimes hear "I want", say in Ireland? Even the the US you could answer "I want to."