When should I take the GRE and when I should apply for admissions ?

Give yourself as much time as possible. I am not an expert on GRE but I have taken the GMAT. Assuming timeline isn’t significantly different for MS or MBA programs, I would say, take the test at least 6 months before you’re ready to apply. That gives you time to either retake the test or replan your strategy as in what schools to apply for, what areas of the application to focus on, etc.

Most of the time, however, you can report a later test date on your application to the school. And when you’re done taking the test, just report directly to the school.