To do well, you're going to need to master both the fundamentals (math skills, grammar rules, etc.) and test-taking skills. If you practice diligently, it can definitely be done. (btw when I took a diagnostic cold, without any prep at all, my score was abysmal too!).
I would suggest starting with an overall review guide that explains the GMAT and how it works (there are a lot of built-in traps). "Cracking the GMAT
" from the Princeton Review
has the best review and tips, IMHO. It also has great practice tests that have answer explanations, and that are closer to the real thing than any other simulated tests out there. Be sure to analyze both your right and wrong answers so you see the patterns. Their tests also break down your score by question type, so you can see your weak and strong areas.
The other materials that most of us consider an absolute must are the official ones...the Official Guide, 11th edition
, a book, and the GMATPrep tests
, which you can download for free from http://www.mba.com
. You can buy the OG and the PR book on Amazon.
If you are still struggling with verbal after you get more into your prep regimen, then I would get a separate verbal workbook. Again I would suggest PR because their verbal prep is particularly good.
Hope this helps get you started...and definitely use the error logs as described on this site!