Hey
Let me help you.

So, basically all the US bschools start the application process usually by Sept/Oct except a few that even start too early.
Every school has some rounds of application. Mostly, R1, R2 and R3. The sooner you apply, the better chances are for you to get selected.
I would prefer you to take the GMAT by may be July/August end so as to give yourself sufficient time to start working on the other components of the application, namely essays, resume, interview, etc.
If you are planning to take the exam by dec, you may be left with very short time for applications.
R1 deadlines are in Sept/Oct
R2 deadline are in Jan first week
R3 is not recommended for International applicants considering very late admission process that may further delay the Visa process.
Feel free to ask any other question you have.
All the best

Now given the time frame till july/aug how can i crack it... I'm suddenly too tensed and not able to think properly. Seriously.