There are a lot of students who used to study for IIT JEE from a very early level. But the point is to understand the JEE problems you will need the specific skills which will grow with time. But as you are ambitious for the case and want to prepare the subject by yourself then you can read the foundation books.

Different publishers like Disha, Pearson etc have foundation books for IIT JEE. It contains the content which will build your concepts for the IIT JEE and will endorse some topics in order to help the students grow there thought process. Also preparing this will help you to crack different exams like KVPY etc if you can build a good basics about.

You can check the books mentioned above by visiting the link below:

https://www.amazon.in/Pearson-Foundation-Mathematics-Class-2021/dp/9390531020/ref=sr_1_16?dchild=1&keywords=pearson+iit+jee+foundation+class&qid=1622903234&s=books&sr=1-16

Also some coaching institutes offers foundational courses like Pathfinder, Aakash, FIITJEE etc.

I hope my answer helps. All the very best for your future endeavors!