As your brother want to appear in TNEA exam for admission in b.tech program and for it asking about Eligibility Criteria, so here I am telling same for you:

TNEA Eligibility Criteria
Students who are the native of Tamil Nadu are eligible to apply for the TNEA.

The aspiring Students whose parents/guardians are working in the public/government sector for the last five years in Tamil Nadu are also eligible to apply.

The applying Students must have passed 12th or its equivalent examination from the well-recognized board with the minimum aggregate of 50% marks (for unreserved category), 45% (for backward class including Muslim), 40% (MBC & DNC) and 35% (for SC/ST/SCA) candidates in PCM subjects.

Keep in mind , a student has to score minimum percentage alloted for his/her community in order for them to get eligibility.
Community HSC (Academic) HSC (Vocational)
General Minimum average of 50% Minimum average of 50%
BC / BC (Muslims) Minimum average of 45% Minimum average of 45%
MBC & DNC Minimum average of 40% Minimum average of 40%
SC/SCA/ST Minimum average of 40% Minimum average of 40%

Allocation of Seats for Engineering Counselling :

Anna university counselling for engineering admissions will be conducted as per the Seat allocation given by the govt .

These are the average percentage a student has to get in Physics, Maths, Chemistry for his/her community .

OC : Min 50%

BC / BCM : Min 45%

MBC / DNC : Min 40%

SC/SCA / ST : Min 40%