How to save tax... even after paying interest for 2 years?

Head under which interest is allowable as deduction under section 24 is "Income from House Property". In your case the house property will come into existance only when the builder has finished the construction and the house is ready to occupy. If your house is ready to occupy then only you can claim interest deduction under section 24 and deduction under section 80C for principal repayment.

If you want to claim deduction for interest payable to your father, then you have to make sure that interest amount is paid to his account and afterwards you can take that amount in cash So that in case of assessement you can easily prove that you have paid interest.