You can make an initial research about which test is more widely accepted in the calibre of business schools which you are targeting. Then, it is best to take the test first and then shortlist the schools. If you feel you can perform better on the test you can take it again. Also, be advised that many business schools will also require an aptitude test such as GMAT or GRE. These two test are more difficult that the language tests (TOEFL, TOEIC, IELTS). It is best that you take an aptitude test as well before you begin your application for admission.
All this said, I would like to encourage you to begin your preparation early. Application deadlines may be as early as 11 months before the start of your studies. Add another 6 to 9 months for test preparation, test taking and work on the application package. This means that it is best to start test preparation at least a year and a half before the intended beginning of your studies. Of course the test preparation and taking can be squeezed in 4 to 6 moths, but this will be very stressful and will not ensure the best results.
Iliana Bobova Head of Admissions Consulting PrepAdviser