Most people start with a practice test to get their current score and see what their strengths and weaknesses are. Most of the test prep companies will allow you to sign up for a free practice test; on my company's web site, go to the store section and select the "free CAT" for $0. Make sure that whatever test you choose gives you a score report with analysis on your strengths and weaknesses.

If you haven't already, you'll also need to do some research on the business schools you would like to attend. Find out what the average GMAT scores are for their incoming students. The difference between your practice test score and the average scores for those schools will give you an idea of how much you need to study.

Then, your strengths and weaknesses on that first test will give you an idea of the priorities to set and the order in which to study.

Most people study for 3-4 months, though some don't take as long and others take even longer. And most people study every day or almost every day; between 1 and 2 hours a day, 6 days a week is best.

Some people study on their own or with friends and others take classes or work with a tutor; you'll have to decide what you think is the best path for you. Most companies also offer free information sessions or the opportunity to attend a free class, so take advantage of those events. You can learn some valuable info and the events will help you to decide whether you would want to work with that company.

However you decide to study, you will need the official materials (the ones that come from the people who make the test). Official Guide 12th edition and the Verbal and Quantitative Supplements are the most recent books. GMATPrep practice tests can be downloaded from www.mba.com (and they're free!). And there is also a quant-only diagnostic test called GMAT Focus. (The books and the diagnostic test cost money; only GMATPrep is free. But, again, these are official test questions from the people who make the test - you really do need to study these questions.)

So, go do two things now:
1) browse the "resources" section of this web site to get some more ideas about what to do
2) sign up for a free practice test with one of the companies (don't use GMATPrep for your first practice test; it doesn't give you any data on your strengths and weaknesses!)

After you've done those things, come back and ask more questions about what to do and how to do it!