I am an ICSE 10th standard student and I want to prepare for IIT JEE exams what should I prepare and how should I prepare

So lets see I started preparing for JEE from 10^thso I believ I can guide you pretty well

1) JEE follows CBSE board so you need to be aware of the syllabus
2) Being in 10^thclass concentrate on your 10^thbut at the same time try to finish few initial chapters (CBSE) of 11^thfor whichever subject you like
3) Assuming you take up a coaching in 11^thand 12^thwhich I advice you should, point 2 will help you to stay ahead few chapters than everyone else and then you can focus on chapters ahead
4) I followed this tactic and it gave me great confidence at the start of my 11^thcoaching and that momentum really helped me crack jee

All the best