I've been corresponding with a student who asked for GMAT and MBA admissions advice. Here's my reply:
I took the GMAT in October of 2005 and applied to Oxford, Cambridge, Emory, and Northwestern by January 2006. I interviewed in February and March and was accepted to Oxford in early April. Thus, if you're just starting the GMAT, I would suggest that you shoot for the 2008/2009 school year, as it's too late to complete the GMATs and put together a well crafted application for the 2007/2008 year (I spent about 4 months on my essays and applications).
My GMAT score was 720 (96th percentile); 45 verbal (99th percentile), 44 quant (73rd percentile), and 5.5 essay (88th percentile). I used the Princeton Review book 'Cracking the GMAT'. I highly recommend this book, as it goes into great detail on the structure of the exam. This is a crucial aspect which is not covered well in most other books. I studied for two weeks, took lots of practice exams, then took the test.
If English is your second language, I would recommend improving your language skills and taking the TOEFL (Test of English as a Foreign Language) prior to taking the GMAT. The TOEFL is virtually always required of non-native English speakers and the practice will greatly improve your GMAT performance.
In selecting a school, it'll be up to you to decide what fits your life best; full-time or part-time, day or evening, distance or class taught, one year or two. For me, the Oxford one-year full-time MBA fit best. I only lost one year of wages versus the standard two year US program, the reputation of the school is excellent throughout the world, and the tuition is relatively low. All in all, it was a great value for the money. Again, though, you must weigh all the factors and decide what fits best for your life.
As for admissions, keep in mind that the admissions process is the method by which schools maintain their quality (once admitted, schools will do everything in their power to ensure that a student does not fail). Three things are critical; GPA, GMAT score, and work experience. If you're weak in one of these areas, then you need to convincingly explain why in your admissions essays. My advise would be to take the GMAT and look at MBA rankings such at the Financial Times and The Economist Which MBA? to get a feel for the average GPAs and GMATs of accepted students before applying.
Good luck!