i am a bit weak in maths mainly in calculus which topics i must concentrate to get around 50 to 65 in maths

If you are weak in calculus then also you can I score good marks in JEE main but not in advance remember it in case of JEE main calculus does not covers most of the person like the advance you can read other topics such as co-ordinate geometry factors probability quadratic equation sequence and series and score good marks because in case of JEE main questions are asked from each and every chapter and you should not leave any chapter if you are weak in calculus you should also cover the chapter switch off list important but asked questions from every year such is logical reasoning.

Here are the chapter wise Weightage of Mathematics Syllabus:

Topic                                       No. of Questions                                 Marks

Coordinate Geometry                                 5                                          20

Limits, Continuity and Differentiability        3                                          12

Integral Calculus                                          3                                        12

Complex numbers and Quadratic Equation  2                                       8

Matrices and Determinants                             2                                      8

Statistics and Probability                                 2                                      8

Three Dimensional Geometry                          2                                      8

Vector Algebra                                                  2                                      8

Sets, Relation and Function                              1                                      4

Permutations and Combinations                          1                                   4

Binomial Theorem and Its Application                   1                                  4

Sequences and  Series                                          1                                  4

Trigonometry                                                           1                                   4

Mathematical Reasoning                                         1                                    4

Differential Equation                                                 1                                    4

Statics and Dynamics                                                1                                    4

Differential Calculus                                                    1                                    4

You can check the detailed syllabus with the given below link.

https://engineering.careers360.com/articles/jee-main-syllabus-weightage

Good Luck!