Hello Razi

You should prepare well the whole syllabus according to previous year question paper weightage.

Mentioned below are some of the best online coaching for IIT JAM

1. GATEIIT     www.gateiit.com
2. Career Endeavour is comes as second.
3. Dips is at third position
4. Chemistry Institute of India
5. Educlasses.
6. T.I.M.E. Pvt. Ltd, Cochin.
7. Brainquest Science Academy, Ahmedabad.
8. I.C.R. Education Services, Bhopal.
9. Medcity International Academy, Kannur.
10. United Studies Education Resource Centre, Gurgaon.
11. The Smart Test Prep, Ahmedabad.

Give mock tests
http://jam.iitb.ac.in/mock1.htm

Kindly visit the mentioned below sites

https://university.careers360.com/articles/jam-mock-test

https://university.careers360.com/articles/how-prepare-jam-chemistry

Hope it helps you.