Sir.

Best colleges offering B.Sc in India take their own entrance examinations which the candidates have to qualify to get a seat in these colleges. These entrance exmas include

BHU UET

JMI Entrance exam

Delhi university

JNU

AMU

IPU

These are some of the premier universities which offer B.Sc.

If one has to take regional colleges' entrance exams then he should study his +2 core subjects deeply, without missing details. Practicing some logical reasoning and english questions will help. Moreover, study some GK from those static GK books which are available at book shops. These things are enough to crack any regional entrance exam. 

Hope this serves your query. Please feel free to respond and ask further questions.

All the best.