In the OG book, read over the Intro to GMAT section. That should explain what each section is about and what types of questions to expect. Then take one of the diagnostic tests bb mentioned. That should give you a rough idea of your current score. Then based on how you do, make your study plan
You can take either GMATPrep or the Kaplan
tests you have in the book or on the CD. GMATPrep will give you a very accurate read on your GMAT Score, but Kaplan
will do job as well by showing you where you have holes. Either option is fine.
You can read more about GMATPrep here: review-gmatprep-gmat-prep-powerprep-power-prep-tests-77548.html
Founder of GMAT Club
Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
Need GMAT Book Recommendations? Best GMAT Books
Co-author of the GMAT Club tests
Have a blog? Feature it on GMAT Club!
Get the best GMAT Prep Resources with GMAT Club Premium Membership