Hi there!
For IIT JAM first of all you need to know the complete detailed syllabus. And mostly the syllabus is from your 3year UG Mathematics Programme.
The syllabus consists of:

* Linear Algebra
* Modern Algebra
* Real Analysis
* Vector Calculus
* Integral Calculus
* Ordinary Differential Equations

Then comes the weightage per topic, the most heaviest of all is Real Analysis and it consists of 32-40 Marks out of 100 followed by other topics.
I'll suggest you to first of all see the previous year papers and try to get an idea about how the questions are asked from a particular topic.

The ODE part is very easy it will ask you to find integrating factors or similar things, so practice ODE thoroughly so that you don't miss any easy question from ODE.
Then comes Real Analysis, you need to understand the concepts of this topic very carefully and revise it on regular basis as it consists of theory. Questions are generally asked from Convergence and Divergence of series so practice more problems of that. It is a heavy one so needs a lot of attention.
Then we have the Calculus part. It is also pretty easy because the Integral Calculus is easy, the problem is Vector Calculus. You need to memorise some formulas here, and their applications.
The Linear Algebra is moderate in exam although the domain of question that will be asked is quiet guessable by seeing patterns.
In Group Theory you will again struggle if you do not understand the key concepts of this. This is a huge topic to be covered or you can say the most lengthy topic in JAM syllabus. So you need to practice the questions more here as it is Algebra in the end.

Prefer good books that will help you out. I'm suggesting some books

* Real Analysis: Bartle & Sherbert, S. Shantinarayan
* ODE: M.D. Raisinghania
* Modern Algebra: Joseph A. Gallian
* Linear Algebra: Schaum's Series, Friedberg
* Calculus: J.Strauss

I suggest Start  preparing  in advance.Practice for Online Virtual Calculator. Syllabus and exam pattern.
Take mock tests .Select best books.Make notes .Practice previous year question papers , Maintain your Health & Remain Stress Free. Learn all  concepts from all the lessons . Good luck .

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