newbie needs some advice

The first thing to do is avail yourself of resources and assess your situation right away. Some tests that you can get a hold of quickly and cheaply are the GMATPrep tests, the six MGMAT CATs that come with their retail books, and the Knewton CAT in our GMAT prep free trial. My honest advice, and this isn't to toot our own horn, would be the take the Knewton CAT first, because that will give you very detailed feedback on the areas in which you need to focus. You can then use the others in the subsequent weeks to gauge your progress.

Once you've taken that first assessment, you need to focus very intently on which areas you can improve in a short period of time. For example, even if you are weak in geometry, don't spend a ton of time on that at first simply because you'll only see around 4 geometry questions on the entire exam. Understand the test blueprint and topic frequency to prioritize your study. Develop a plan on how many hours you will spend each week, what resources you have available (books, online, etc.), and intensely study every exam you take. Focus on every question, even ones you answered correctly, to ensure you understand every bit of the concepts involved. You'll never see any of those exact questions again, but ones very similar -- make sure you have an understanding of the situation so you can apply it in other cases.

As you get a week or two in, continue to use this forum to ask questions, hone your study plan, etc. -- we're all glad to help!

A+ response from Jose in my opinion.

I would only add that it's important that you break your study into subtopics. For instance Geometry -->Triangles vs Geometry --> Coordinate Geometry. If you can find a way to track how you're doing in each little piece of the GMAT, you'll have a better chance to spot opportunities for rapid improvement.

If you've always been a poor test-taker even as you've been an excellent student, I guess the million dollar question is why? I can't pretend to know you but if the answer has anything to do with testing anxiety, consider reading the document I"ve attached to this message.

Take practice exams often and as Jose said, scrutinize your performance in each. The ManhattanGMAT exams are really good at breaking down performance by subject matter (I have no experience with Knewton exams).

As you get within 3 weeks of test date, stop taking 3rd party exams and just take GMATPrep tests (available for free from mba.com). They are much harder to scrutinize because not only does the software give you no per topic analysis, but it also gives you no solutions/explanations. You can get explanations by asking around forums where you'll find a helping hand more often than not or from a 3rd party like GMATFix which offers solutions to most GMATPrep quant questions.