I want to first say good luck.
GMAT is a nerve wrecking experience for many. There are some special people who can study for a bit, take the test and walk away with 750+ but I was not one of them unfortunately.
The first place to start is to take the free Manhattan GMAT
or Knewton test. I haven't taken the Knewton test but I have heard many good things about the test and I can tell you for certain that Manhattan GMAT
test will give you a pretty good sense of how the test structure will be like and what kind of questions you can anticipate.
After the practice test, if you are significantly below(100+) your desired score, I'd say either postpone the test or cancel the test because you'd need 3~4 months to increase significant amount of points.
If you are completely unprepared, I'd suggest taking a GMAT prep course.
Good luck with your journey!