بسم الله الرحمن الرحيم
You will have to pay 300 rupees as zakah on 12000 rupees. However, zakah is paid as per lunar calendar. Thus, mark the day of the lunar date on which you became sahib-e-nisab and pay your zakah on the same date each year.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.