I am in 10th std. Should I start preparing for IIT JEE?

There is no wrong if you go for any coaching for JEE from now. At Max I recommend you to spend 10- 15 hrs a week and not more than that. The most important is your 10th board exams. Make sure your preparation for board should not be affected by your coaching class.

Get yourself ready but don't start right now ,because you have to clear class X exam also with decent score.

Advantages of preparing for IIT JEE from Class 9th and 10th

1. Extra edge over peers "? Students studying for JEE since Class 9 are bound to have extra advantages over their mates. The expert created helps them understand the concepts in a better way. Preparation from starting gives them ample time for revision and practice. By the time they reach their Class 11th and 12th, they have revised and practiced the important concepts enough number of times.

Physics

The topics you study in these classes will help in strengthening your foundation for more difficult and complete topics. Some of the Class 9th and 10thtopics in common with JEE Main Physics syllabus are as follows:

Motion
Force and Newton's Laws
Work, Energy and Power
Gravitation
Properties of Solids and Liquids (Matter "? Its Nature and Behavior)
Sound
Electric Current, and Magnetic Effects of Current (extension from Effects of Current)
Optics (extension from Natural Phenomena)
Atoms and Nuclei (extension of Structure of an Atom)

Chemistry

A few of the Class 9thand 10th topics in common with JEE Main Physics syllabus are as follows:

Matter and its nature, and States of Matter
Atomic Structure (extension of Structure of an Atom)
Chemical Bonding and Molecular Structure (extension of Chemical Substances "? Nature and Behavior)
Environmental Chemistry (extension of Our Environment)

Mathematics

Some of the Class 9th and 10th topics in common with JEE Main Physics syllabus are as follows:

Quadratic equations,
Co-ordinate Geometry,
Probability, and
Trigonometry (especially Trigonometrical identities and Heights and Distances).