If you want to pursue pure sciences then the best option is to enroll in an integrated program of 5 years, as job prospects are brighter for a Masters degree holder. Most well known colleges have integrated programs and entrances for them. For an IIT you have to crack JEE Advanced and for NITs, JEE Mains ( a mediocre rank will also do, as students are more interested in getting an engg degree. ) Else, get into IISERs or IISc. For IISERs, get a good rank in JEE Main or score excellent in boards, write an aptitude test and get in. For IISc, crack KVPY entrance exam or AIPMT/ JEE Main/Adv with a good rank.
All the best 🙂