GMAT is not really tough at all but its a bit chellenging. it is required meticulousness, dedication and planning. There are following tips and strtgeies that will help you in getting good score -
Make yourslef familiar with computer: As this is lengthy computer based test, so one should get familair with computer typing.
Do online pratice from: Do some online pratice such as various realted quiz and othr things.
Use process of elimination: One should be aware of how you can eliminate an obtion which is not within the scope, too extreame. It is far easier to eliminate wrong answers than it is to pick the right one. So, when you are not sure about answer choices on a given question, try to eliminate all the wrong ones first. Make a case for why each one is wrong. The choice that is the best to disprove will likely be the right answer.
For Sentence correction: Go through the fundemantals of Grammar which will help you eliminating the answers. Go through concise and short answers if you dont have any other valid reason to eliminate.
For reading comprehensions, read the passage first: When you come across a passage based questions, read the passage first, not the question. This is generally believed that the better strategy for two reasons. First, you can only see one question at a time, but there will be three or four questions for each passage. Second, even questions that seem to be about a small detail or sub-topic will require a holistic understanding of the passage to answer correctly. You actually have a better chance of identifying the right answer in the majority of questions if you aren’t biased toward one detail or sub-topic, but are reading for the main idea instead.
For crtical reasoing: Read the quetsiobn stem first then the stimulous as question stem will guide you where does this question belong to each categories of question completely different approach.
For gmat Quantitative aptitude questions get your fundamentals cleared which will help you in getting good score.
Best of luck!