You can go to IIT's/NIT's by qualifying IIT JAM exam. So year after BSc you can obviously join IIT's/NIT's. For MTech you need to have a MSc , after MSc you can pursue MTech by qualifying GATE and you can get admitted in different uni's including IIT's.

Also in some courses you may get a chance of doing Integrated PhD/MSc-Mtech course after qualifying your IIT JAM exam.

Wish you a great eve!