Engineering

I'm assuming you're asking (How to get admission to NUS? ). Okay so, unlike the United States, Singaporean admissions are not holistic, and lay great emphasis on school grades. Depending on what board you belong to, the NUS has a specific cutoff or limit for consideration which will be easily available on their website. An estimate would be at least 90% marks in the final examination in whatever board you come from, backed by consistent academic performance in the past 4 years. (For undergraduate, that'd be freshman year, sophomore year, junior year and senior year, or in India, 9th grade onwards until 12th grade. )
Hope this helps. Feel free to ask follow ups if you have further questions.

No, there is no seat reserved for any of the Polytechnic colleges to take admission in COEP for direct 2nd-year degree courses.
The admission procedure is as follows
Your percentage score of Diploma is the only criterion for admission to COEP. Of course, there are Caste based reservations in these seats as well. If a branch has an intake of 60 seats in the 1st year then there are 20% of seats added in that class at the start of 2nd year. This makes 12 seats for Diploma graduates in a class having a strength of 60.
For COEP Mechanical, there are 120 seats at 1st-year intake so there are 24 seats (20%) at the start of 2nd year for Diploma students.
Here is the link you should refer for college-wise cut-offs, deadlines, available seats, and all the things related to Direct 2nd-year admission to Engineering courses.