When do i need to make a repayment to HBP ? and how its calculated ?

You will not need to show a repayment until filing your 2017 tax return.

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/rrsp-reer/hbp-rap/rpymnts/menu-eng.html

Thanks for your reply, Just wanted to confirm, so the repayment starts only after i file tax for 2016 ?

The way it works is you purchase your RRSPs as normal...when you file your 2017 return, you assign a portion as repayment and you can still claim the deduction on the balance.  I would recommend showing the minimum repayment required, and maximizing the deduction for the tax year...you have 15 years to pay the amount back, and it's interest free.

your NOA will show you the required minimum repayment for the following year.

Here is some additional info: https://turbotax.intuit.ca/tips/repaying-withdrawals-under-the-home-buyers-plan-5216
To be clear you make your RRSP contribution normally and you would claim an amount as a HBP repayment, thus not affecting your RRSP deduction limit.