Also, just as a side note, you don't HAVE to take the GMATprep test to start off. It is good to have a starting point so you know how much studying you need to do, but it can be from a test prep company also. I only took a test prep company series of tests, 7 in all including a diagnostic. Started with 570 and made my way to 690 on last practice test and then threw down a 710 on test day. I never took GMATprep, just did the practice question in the program to familiarize myself with the interface. I would encourage you to save atleast one of the GMATprep tests to take about a week or so before the big day (I wish I had taken one just to see where I stood, but my best ROI of time was not in a practice test at that point). The big problem with GMATprep is it will give you the best idea of where you stand NOW, but really won't help too much with your studying and moving forward. Description of missed problems aren't comprehensive, and there are no other statistical analysis of your performance. Just your score, and make sure you write it down b/c you only get to see it once. Best of luck with your studying!
bb's post with the study plan is by far the best resource to get you started. Its what I used, and pushed me the right direction all the way.