Hello dear

It's too good to start to prepare early from your school days .if you are in class 12 than u have to prepare for your board exam soo . U must have to maintain balance between them because if u score 75% in CBSE board aur top 20percentile in your respective board than u are able to fill to get admission in nit college and iit college.

Soo first u must focus on your school NCERT books

.for chemistry and physics  your NCERT is enough read them as much u can and just solve the question related to them.and don't forget HC Verma

For mathematics I suggest u to follow Rd. Sharma for board and cengage books for jee.

Thats enough if u want to score better than u must practice too much questions .

Now your answer which topic

And don't want u to bond that folllow some certain topic every topic is important and concept related.soo don't take this decision that u have to follow this topic only . Prepare every topic and in last u will get which is easy to prepare and in which u will score more.