Have you given any of the official GMAC mock tests? see how you score in these. Then take up the questions that you did not get right ( you can review the papers after you have given the tests) and go through whatever materials you have to find a similar question and the plausible explanation to the answers. If possible, try the same strategy for some other mock exams as well. That way, you can pinpoint your weak areas and improve in the same. No matter what others say, if you have decided that GMAT is the way forward in your career, go ahead and sit for the exam. There are numerous examples where people have score way more that what they have scored in their mocks. So don't get disheartened, just give your BEST !
All the Best for GMAT !