Hi amanpreet0072,
Many Test Takers spend 3 months (or more) on their studies before they hit their 'peak' scores, so you should consider that when planning out your studies. The GMAT is a big Test, but it is predictable and standardized, so you CAN learn all about the content and patterns, and you can train to score at a higher level.
Since you're not planning to apply to Business School until 2018, you have some time to work on the GMAT 'piece' of your application, but when you do decide to start studying, you have to be ready to commit to a consistent study routine for a significant period of time. The 750+ score is about the 99th percentile, meaning that 99% of Test Takers cannot score at that level.
1) Have you thought about when you might want to take the GMAT?
GMAT assassins aren't born, they're made,
Rich