CBSE 12th

I would say first of all focus on your 12th board, you need above 55/60% in physics, maths and english for certain flying schools. Its good that youve decided it in 12th itself. If you get time during your 12th, I would suggest first of all undergo your class 2 medicals, it won't take much time, about 56 hours depending on the hospital. Here is the list of dgca class 2 medical examiners, http://dgca.nic.in/medical/class.check whichever is the nearest and take an appointment. But do check the validity of class 2 medical, Im unsure. Meanwhile, check out flying schools in India, here is the list of a few good ones. IGRUA, Rae Bareli (Indira Gandhi Rashtriya Udaan Akademi)
CAE (NFTI) Gondia (National Flying Training Institute)
MPFC Indore (Madhya Pradesh Flying Club)
Bombay Flying Club
Rajiv Gandhi Aviation Academy, Hyderabad
Chimes Aviation
Academy Of Carver Aviation
There are more flying clubs. So, I would suggest pick a sunday/ upcoming summer holidays to surf all their websites and talk to the ones you shortlist. Know about their entrance exams, if any, fees, when are you eligible for exams etc. I would personally suggest first 2 flying schools in the above list IGRUA and CAE Gondia, both of them charge higher than all others but the quality of training is amazing. Do check out their websites. They both conduct entrance exams thrice an year but I think you'll be eligible to appear only after you get your board result. After finalising the flying school, get to know about their entrance exams, talk to people who are studying there or have given these exams. Start preparing for their entrances before or after boards, that's up to you. After your 12th boards, apply for class 1 medical appointment, undergo it. It takes a few months to get the certificate. Read the medical procedures at DGCAs website. There is also a Cadet Pilot programme run by CAE Gondia and IndiGo in which you have job security post training. Check that out too, its expensive but there is job guarantee. Make wise choices.
Good luck.