You need to qualify entrance exam exam such as IIT_JAM, NEST etc. in order to get seat in an indian university. The best universities  are: Delhi University, Delhi

Jamia Millia Islamia,

JwaharLal Nehru University,

Lovely Professional University Jalandhar 

University of Pune,

Dr.Bhim Rao Ambedkar University, Agra

Banaras Hindu University

Anna University

Christ University,

Savitribai Phule University

The Maharaj Sayaji Rao University,Jadavpur University, 

University of Hydrabad.

Hello Shahnaz,

For Indian universities:

If you are not from B. Sc Math, first check whether the university offers M. Sc math to such candidates.

Then, download and practice mock papers for its entrance exam. 

After you qualify and sit for the exam based on your Bsc syllabus, you can get admitted.

For foreign:

You have to take language tests IELTS or TOEFL.

Also, write an SOP (Statement of purpose).

Then, you have to take an exam like the GRE/SAT as per the requirements of the specific university.