Dear ynyus75,
Thanks for your post! While there are likely numerous places you can find this information throughougt this forum and across the web, here's a basic summary of how the GMAT works:
1) There are three parts to the GMAT: the analytical writing assessment (AWA), the quantitative section and the verbal section. The AWA requires that applicants draft two essays. The other two sections are multiple choice.
2) Test takers receive a score in each section - with the AWA being scored on a scale of 0.0-6.0 and the other two sections being scored on a 0-60 scale. All of these 'raw' scores are converted to percentiles so that applicants (and schools) can see how you compare with other test takers. For example, someone who scores a 50/60 in the quantitative section is likely to be at around the 95th percentile on that section (e.g. top 5% of al people who have taken that section of the exam).
3) Test takers also receive an overall GMAT score on a scale of 200-800. The overall score is determined by the test taker's raw results on the quantitative and verbal sections of the exam but NOT the AWA.
4) As you may have gathered from the way the test is scored, the schools place an emphasis on the quant. and verbal sections much more so than they do on the AWA.
As to what sort of score one should have for a top-20 MBA program, this varies somewhat from the top-5 down to #20, but in essence you should be aiming to score around 700+ on the exam. You should also be aiming to surpass the 80th percentile in both the math and verbal sections. The average GMAT score at places like Wharton and HBS 710-715 these days.
In terms of prep time, six weeks is a good start - but it all depends on how much time you intend to put into it during the six-week period. In essence, you'll need to do a couple of hours of problems each day, a full-length, timed test each weekend, etc. I strongly suggest that you read through the resources in this forum, since there are hundreds of pages of advice in terms of how to prepare, how long it takes, etc.
Best of luck,
Graham