To obtain the Commercial Pilot Licence (CPL) under Directorate General of Civil Aviation (DGCA) in India you traditionally need to have passed Class 12th with Physics & Mathematics (PCM). If you did not choose the science stream you'll need to make up for that.
Immediately after Class 12th: As soon as you finish 12th (even if it's commerce/arts stream) you should check if you have PCM. If not you should enrol in open schooling (such as National Institute of Open Schooling or an equivalent board) to clear Physics & Maths as private/extra subjects.
As soon as you clear Physics & Maths: Once you cover those two subjects you become eligible to apply for pilot training ground classes and flying school. The idea is to avoid delay the sooner you satisfy PCM eligibility the sooner you can start.
Ensure age & medical fitness are ready: You must meet age minimums (for example 17 years to begin certain pilot training) and pass Class 2 / Class 1 medical fitness.
Start training soon after eligibility: Once you are eligible (PCM + age + medical) you should join a DGCA approved flying school. Delaying too much means you lose time in building flying hours and experience.