Hi Minh,
From your initial post, It sounds as though you're just beginning your GMAT studies. Is that true? Have you done any studying so far? When it comes to studying for the GMAT, there are a variety of different resources that you might use. Since the Exam is standardized - meaning that the content is consistent and you can learn the Tactics, patterns and little 'secrets' to the Exam - you don't necessarily need a 'mentor' to properly train to score at a high level.
If you're just beginning, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can access 2 for free from
www.mba.com (and they come with some additional practice materials). If you want to do a bit of practice first, so that you can familiarize yourself with the general content and question types first, then that's fine - but you shouldn't wait too long to take that initial CAT. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here (or you can feel free to PM me directly) and we can discuss the results and come up with a study plan.
I'd like to know a bit more about your timeline and goals:
1) What is your goal score?
2) When are you planning to take the GMAT?
3) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich