To find out exactly what the GMAT is, what it tests and how scoring is done, please read this link. http://www.gmac.com/gmac/thegmat/
Briefly, the GMAT tests on two areas: Quant and Verbal (excluding the AWA essays you will need to write before the quant and verbal sections). Quant touches on subjects that you learn in high school: Algebra, Number properties, Geometry, Ratios/Fractions, Divisibility tests, Combinatorics etc. You will want to brush up quickly on these areas. The quant section is broken down into problem solving and data sufficiency. You can read more about these in the link I gave you.
The verbal section is broken down into three areas: Sentence Correction (tests grammatical skills), Critical Reasoning (Logical/Critical thinking) and Reading Comprehension. For grammar, you might want to get the MGMAT SC
guide. The reviews are good. The Kaplan Verbal workbook
seems to be good as well. It will help you get accustomed to the verbal skills needed quickly.
At the minimum, get the Official Guide (11th Ed or 10th Ed). Work through the guide (25PS, 25DS, 10SC, 10CR and 3RC each day should allow you to finish up the OG in about 1 month). What you should do initially is to do it untimed. After a while, when you're more used to the problems, start to do the sets in a timed condition. The GMAT is timed, so better to get used to it earlier. Read the solutions thoroughly after each set, make sure you understand them. Keep an error log
of the mistakes you make, it will come in handy when you start your revision prior to taking the GMAT.
Do not forget the GMATPrep software. It's almost a must - gives you a feel of the desktop you will see on test day, and get some practice in a timed environment. Do not skip the AWA section for any CAT practice tests you do, this will give you an idea of how much energy you will really need to complete one CAT.
So where does GMATClub come in? You can read through the quant/verbal forum
and look at suggestions. Participate in the discussions and learn from others. You can post your questions here as well if any explanation you come across in the guide is not clear enough.
That's about all I have for now.