So it mean I can pay 0 fee everytime I send BTC and it still get confirmed one day?
It is entirely possible. So long as a transaction is broadcast, there is always the possibility for it to be confirmed.
I was using MultibitHD do sending payment. And I am not sure how to find my private key or how to double spend using this wallet.. Please help
IIRC MultiBit makes it difficult to do advanced Bitcoin stuff. I think there is an option to clear all unconfirmed transactions (repair wallet or something like that) and then you can make the double spend.