Hi congchualon,
If you currently have a list of Schools/Programs that interests you, then you can research those Programs in more detail - most School's websites will include information on Average GMAT/GRE Scores, "middle 80" Scores and/or Score 'ranges' for recent classes. That data will give you some idea for the type of goal score that you might want to aim at.
It sounds like you're just starting out. Is that true? Have you done any studying so far? If you're just beginning, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can access 2 practice GMATs for free at
www.mba.com and two practice GREs for free at
www.gre.org. 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 and we can 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/GRE?
3) When are you planning to apply to Business School?
GMAT assassins aren't born, they're made,
Rich