Pratik, a few years ago it was not possible to do graduation in commerce being a science student. The eligibility for a student to choose BCom or BBM, or commerce-related degree programmes, he/she should have studied commerce and accountancy in Plus-Two but in recent years many universities have taken this initiative to allow a science student to do BCom. Colleges are open-minded about admitting students from the science background. So, you can do B. Com. The Eligibility criteria are:- 
A.Candidates from Commerce Stream- Minimum 60% aggregate AND 55% in Accountancy
B. Candidates from Science Stream- Minimum 60% aggregate AND 60% in Mathematics
C.Candidates with Eco, Stats, Maths in Class XII- Minimum 60% in aggregate AND 60% in Mathematics

Colleges that can be considered are: Hans Raj College, New Delhi, Madras Christian College, LovelyProfessional University, Hindu College, New Delhi, Christ University, Bangalore, St. Joseph’s College of Commerce, Bangalore, Shri Ram College of Commerce, Loyola College, Chennai, Narsee Monjee College of Commerce and Economics, Mumbai, St. Xavier’s College, Mumbai, K. J. Somaiya College of Arts and Commerce, R.A. Podar College of Commerce & Economics

Hello,
Yes you can apply for b.com course after class 12th science.Here is the eligibility criteria, which has been decided by educational committee-

• B.Com (Honours) ALL BOARDS:

(I) Candidates from
Commerce Stream- Minimum 60% aggregate AND 60% in Accountancy
(II) Candidates from
Science Stream- Minimum 60% aggregate AND 60% in Mathematics
(III) Candidates with
Eco,Stats,Maths
in Class XII- Minimum 60% in aggregate AND 60% in Mathematics

• SC/ST (ALL BOARDS):

(I) Candidates from
Commerce Stream- Minimum 40% aggregate OR 40% in Accountancy
(II) Candidates from
Science Stream- Minimum 40% aggregate OR 40% in Mathematics
(III) Candidates with
Eco,Stats,Maths in Class XII- Minimum 40% aggregate OR 40% in Mathematics

• DIRECT ADMISSION

B.Com (General)(All Boards) :-
(I) Candidates from
Commerce Stream- Minimum 60% aggregate AND 55% in Accountancy
(II) Candidates from
Science Stream- Minimum 60% aggregate AND 60% in Mathematics
(III) Candidates with
Eco,Stats,Maths in Class XII- Minimum 60% in aggregate AND 60% in Mathematics

• SC/ST Candidates:-

(I) Candidates from
Commerce Stream- Minimum 40% aggregate OR 40% in Accountancy
(II) Candidates from
Science Stream- Minimum 40% aggregate OR 40% in Mathematics
(III) Candidates with
Eco,Stats,Maths in Class XII- Minimum 40% aggregate OR 40% in Mathematics
Although it can vary with different colleges.But approximately it is same.
Good luck.