You appear not to have done any research, on this forum at least.
Do that first.
Also check this out:
http://www.sfcpl.com/
Then finish your NS, go to university.
ONLY after that think about being a cadet flying instructor. You will NOT be an SIA instructor, but an SFC instructor when you complete your training. You will teach cadets how to fly SFC aircraft. Big difference from being an SIA instructor teaching second officers / first officers / captains.
From time to time there will be advertisements in the Straits Times looking to hire cadet flying instructors.
Anyway, you have many more years to go. One step at a time.