i just came to class 10 and now i am thinking to start iit preparation from now but i don’t know how to start

i don;t go to coaching anywhere

where should i join?and how to study at home?please explain briefly

You can join any coaching pre-foundation course which you find good. Start with class 10th books and build concepts on that base.

You should start studying right from 11th itself. Join some nearby coaching where you understand your teachers who focus on IIT-JEE. After coaching, self study is also necessary at home. Without self study, you will not be able to understand the concepts.
Ask your friends where they are taking their coaching. As you havent mentioned your city, i cant tell about the coaching centers in your city.
All the best.
Thanks