Probably everyone looking over this post would have a fair idea about what GMAT is all about. If not then have a look at this. If GMAT is your god as of now, then Official Guides are the bible. OG guides are released by GMAC, the same people who creates the test. You can get these here. Everyone who prepares for GMAT should have it for your preparation, every other material you can get in the market are the ones with which you can supplement your preparation.
How to start off ?
Which topics can I study now ?
So many topics are there in book ... I am confused ...
Should I start solving problems already ?
If these are your thoughts, then you have come to the right place. Start off with the concepts available in OG. OG does a good part in reviewing the basics, if you find the concepts to be too difficult to grasp, then you should understand that your math is rusty and you need to review the basics. Few suggestions to learn the basic math. The order or priority is directly proportional to the quality of the book.
1.Foundations of GMAT Math by MGMAT
2.Kaplan GMAT Math foundations
3.NOVA GMAT prep course
Coming to the verbal part, you see a question something similar to this
Which of the following, if true, most strongly supports the view that it would not be advisable to try to eradicate agricultural pests that go through a caterpillar stage by spraying croplands with the enzyme mentioned above?
and feel "Ehhhh !!! What is it ? What am I supposed to do ? Come again ...", scratching your head
You shouldn't face a similar situation. You need to have a fair idea of what is asked and how to solve each type of question. What you need to look for is
What type of question it is ?
What is SC ?
What you are supposed to do when you see an SC question ?
What is a CR ?
What are you supposed to do when you face a CR question.
Your initial knowledge should be limited to that. Your only choice is OG, do not go for any other materials right away. Start reviewing whatever concepts given in the OG. You probably need improvement in all the three sections, but need not bother, you have just started your preparation and you have not even solved any problems as well. So reviewing basic stuffs should take you close to 2 or 3 days. So far I haven't said anything about solving problems, there is a reason behind it. You need to know where you stand, you need a baseline estimate of what potential you have.
There are few reasons behind, why you should also have gone through the basics. Your baseline test shouldn't be a wasted one, as in you shouldn't get stuck and waste time in finding out how to solve a Critical Reasoning problem or what to do for a Data Sufficiency problem. Also you shouldn't have solved all or most of the problems in OG and then take a test, you would have wasted a sufficient amount of problems.
Now you can go for the test, you should take the GMAT Prep test 1. Take the test with strict official condition. The GMAT tests not only your problem solving skills but also how you react under pressure/strain. You should have good stamina to sit for 3.5 hours. Skipping AWA or IR is of no use as you will solve verbal question an hour earlier which will inflate your scores. Its better to start off with GMAT Prep tests rather than other tests, because only GMAT Prep is the best available test to estimate how much you will score in the actual test. Majority or even none of the test prep company materials are not as close as GMAT Prep in imitating the GMAT style of questions. So the sooner you gauge yourself, the better you can prepare.