its ,good you thought of preparing IIT/JEE  preparation

first of all it depends on the university you choose like ex :if you are  state board then you have to cover more topics to study ,and if you are in CBSE ,then the 11th syllabyus portion is the key fundamentals which ou want to concentarte ,

and side by side you have to prepare by taking any kind of online preparations or offline courses.

You should join coaching institutions like PACE, RAO, Resonance, Aakash whichvare topclass for Jee Advanced preparation.

Start with understanding all fundamental and go for NCERt boks to prepare for JEE Main first.

Once you qualify under Jee Main you are suited for Jee advance.
You need to focus on weak areas and give nore than 30 mock exams to identify where you need focus.


https://www.askiitians.com/iit-jee/how-to-prepare-for-iit-jee.html