First of Congratulations for scoring 81% in cbse board..

If you want to get admission into Thapar University then there are 2 modes through which they take admission..

1. First Mode is through JEE MAINS ( 70% of the Total  Seats are occupied through this mode) and the basic eligibility/Minimum requirement for Boards is with at least 60% marks (55% for SC/ST candidates) in aggregate of three subjects, namely, Physics, Mathematics and any one subject out of Chemistry, Biology, Biotechnology and Technical Vocational subject..

2. Second mode is directly through board percentage ( 30% of the total seats are occupied through this mode) and the basic eligibility is that  student has passed 10+2 or equivalent examination with at least 75% marks (70% for SC/ST candidates) in aggregate of three subjects, namely, Physics, Mathematics and any one subject out of Chemistry, Biology, Biotechnology and Technical Vocational subject.

For further details, you can go through their website (link is shared below)

http://www.thapar.edu/admissions/pages/eligibility

I hope your doubt is cleared ..Thank you