Hi Kritika,
Quite an interesting field you are opting for I must say. BYNS is a 4.5 year long undergraduate programme, where a 1 year internship is compulsory. Speaking of some top institutions, there are
1. SDM college of Naturopathy and yogic science
2. CMJ University, Shillong
3. Sai Nath University, Ranchi
For BYNS the fees ranges from about INR 10000 to 60000. And the eligibility criteria is to be qualified from 10+2 from a recognised institute.

Good Luck