Hi Bowo,
While many of the 'success stories' written in these Forums are from Test Takers who were already scoring at a high level in the Quant section, there are lots of Test Takers who are in the same position that you are in, so you're not alone. The GMAT is a predictable, consistent Exam though, so you CAN train to score at a higher level. From what you've described about your studies so far, you haven't put in the necessary time/effort or worked with the proper resources yet.
1) Have you taken any FULL-LENGTH CATs (with the Essay and IR sections)? If you have, then how did you score? If you have not, then you should take one soon (you can download 2 for free from
www.mba.com).
2) What is your goal score?
3) When are you planning to take the GMAT?
GMAT assassins aren't born, they're made,
Rich