Firstly you need to specify in which college and in which particular stream you want to take admission for doing b.sc.
If you aspire to take admission in top government and government funded colleges like DU or PU you need to score really well in your class 12 examinations.
Talking about Delhi University if you belong to general category you need to score really high if you aspire to take admission in top most colleges of the university atleast score 95% in your board examinations.
If you aspire to take admission in PU then you need to get a good percentile in PUCET examination and you also need to score good in class 12 because it has some weightage too. If you score above 90% in class 12 and have a minimum percentile of 95% in PUCET examination then you can get seat in the main branches till the last round of counselling considering you are in general category and have no quota like rural area quota etc.
Every state conducts its own examination (CET) to get students admitted in b.sc government colleges and other colleges registered. Pass in every subject atleast score 50% to be eligible for b.sc .
Go for private colleges if you fail to get seat in government colleges b.sc is not a degree too costly.