The best way to start your test preparation is to check your level.
Step one can be a sample/mock test or parts of the test. Firstly, familiarise yourself by doing a sample test: not necessarily a full test, but elements from each section.
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The second step will be to take a diagnostic test at a test preparation centre. The benefit of this is that you get feedback from a tutor and can discuss your preparation strategy. You may first need to improve your knowledge (mathematics for GMAT and GRE) and language skills (grammar, vocabulary) before you start focused preparation for the test itself.
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It is advisable to discuss your preparation strategy with experts and tutors who are professionals in test preparation and who know your education background. Some concepts in the test, especially in GMAT and GRE, are very different from the way they are taught in high school or university. This applies not only to specific terminology in English, but also to understanding a very different approach.
When you discuss the tests and test preparation with friends or students who had already taken them, remember that their opinion is subjective and that you probably have different starting levels and learning styles. If you are talking to international students in forums and blogs, bear in mind that the level of knowledge and skills in different countries varies due to differences in their educational systems and approaches.